Driver Store Explorer loads all the cached drivers which you can filter by device type, manufacturer, driver version and even the size of the file, making it easy to find large drivers worth deleting. There's even a "Select Old Driver" button that will auto-check the boxes next to unused driver files, which can be deleted all at once.
Cleaning the DriverStore FileRepository folder with Driver Store Explorer released 2.6GB of space, most of which was Nvidia drivers...
C:\Windows\WinSxS - Occupied 7.56GB
WinSxS is the location for Windows Component Store files, which are used "to support the functions needed for the customization and updating of Windows," including operating system functions such as Windows Update installing new component versions, uninstalling a corrupt update as well as other system recovery operations. Microsoft has instructions for cleaning the unused components in this folder, which involves entering a command or two in an administrator Command Prompt.
The company says that the following command will remove previous versions of updated components, but when we tried this, the operation gradually occupied around a gig of space and then released that gig after completing without freeing up much if any space.
/online /Cleanup-Image /StartComponentCleanup
However, adding /resetbase to the end of that line (so, /online /Cleanup-Image /StartComponentCleanup /resetbase) will remove "all superseded versions of every component in the component store" and this managed to reduce the size of our WinSxS folder by nearly 2GB from 7.56GB to 5.98GB.

C:\ESD - Occupied 2.68GB
Microsoft recommends against deleting this folder because it's "used by Push Button Reset feature to reset your computer back to its original install state," in other words, files used for resetting Windows. That said, the folder can technically be deleted if you don't plan to use this feature or have already made an external USB recovery drive (search Start for Create a recovery drive) which will provide the same functionality.
Note that secondary system files scan in Disk Cleanup in Windows 10 may include the ESD folder as an option for deletion. We're doing the cleanup on Windows 8.1, but ESD can still be included with the instructions in the Disk Cleanup section above.
If you can't see the folder while browsing C:\ in the Windows File Explorer, go to View and check the box that enables "Hidden items."
Our machine did not need the extra space badly enough to delete the ESD folder.
C:\Windows\Installer - Occupied 1.1GB
This directory serves as a cache location for Windows installer based applications along with "stripped down versions of the Windows installer data files," according to Microsoft.
During application install, update of the application or application removal, this directory is used by the application to confirm the existence of previously installed items to determine the next steps the installer needs to take.
The company suggests that simply uninstalling a given application is the best way to clean its associated files from this folder.
C:\Program Files\Nvidia Corporation\Installer2 - Occupied 1.1GB
This location contains installation files from previous iterations of Nvidia's drivers and the company's web page about this folder says the contents of this folder can be deleted without affecting your currently installed drivers or software. "At most, it will prevent complete installs from occurring in the case of using an older driver from the OS driver store." We deleted all of the files in Installer2.

C:\Users\username\AppData\Local\Nvidia\NvBackend{ABAF8EFC}\ - Occupied ~1GB
Nvidia's "NvBackend" folder took up nearly a gig, 830MB of which was in "Packages" and on our system this included 114 executable files named "DAO.insertrandomnumber.exe" that occupied 512MB of space. After much searching we couldn't determine if these files were safe to delete.

Proceed at your own risk...

7.Reduce Windows System Restore, hibernation & pagefile

These are the most extreme methods for recouping disk space on a Windows installation. While not recommended prior to trying the previously listed options, many gigabytes can be salvaged by making some advanced system tweaks.
Adjust your System Restore settings
Introduced with Windows ME, System Restore creates automatic backup files that can return your system, settings, programs and Windows Registry to a previous state. Depending on the size of your drive and the capacity percentage that System Restore is set to occupy, you may be able to reclaim several gigs by reducing the amount of storage it can use, deleting old restore points or disabling the feature altogether. To get started:
  • Search Start or Run for sysdm.cpl
  • Go to the System Protection tab
  • Select your Windows drive and click Configure...
The window that opens will have a slider that you can adjust to change the maximum disk space that System Restore is allowed to consume on your drive -- the less space you define, the more frequently old restore points will be deleted. You can also manually delete old restore points or disable the feature from the same window.
Restore points can be upwards of a gigabyte so somewhere around 2GB is probably the minimum you'd want in most situations, although we've been using 1.49GB (2% of our drive) for years with no issues. This seems to provide enough space for at least one restore point, which has spared us the trouble of reinstalling Windows once over the years.
Delete hiberfil.sys, disabling Windows hibernation
Hibernation writes your current system state to a non-volatile memory source such as your primary HDD or SSD, allowing you to turn off your system and restore your progress at a later date.
Turning on a system from hibernation is generally faster than starting from a cold boot and it doesn't require a constant power source like sleep, which stores data in RAM.
That said, if your operating system is on an SSD, load times should be sufficient and desktops are usually connected to a power source anyway, so using sleep instead is typcically fine.
Your hibernation file (hiberfil.sys) is probably occupying a few gigs to a dozen or more and the file can be purged from an elevated Command Prompt:
  • Search Start or Run for cmd.exe
  • Right-click on cmd.exe and choose Run as administrator
  • Enter the following command: powercfg.exe -h off
We already had hibernation disabled but re-enabling the feature consumed 12.8GB on our SSD.
Reduce or relocate your Windows page file
While not encouraged, you may be able to shrink or disable your page file (pagefile.sys), which allows Windows to use some of your drive space as system memory. The operating system can move infrequently used data from your RAM to the page file on your drive and the feature is particularly useful if you are running low on system memory.
By default, the Windows page file is configured to occupy many gigabytes on your system drive (3 x RAM or 4GB, whichever is larger) but if you have plenty of RAM or are confident that your system's performance won't be affected by reducing the size of your pagefile, the feature can be tweaked by navigating through these menus:
  • Search Start or Run for sysdm.cpl (or right click This PC > Properties > Advanced system settings)
  • Select the Advanced tab and then open Performance Settings
  • Go to the Advanced tab (again) and click the Change button under Virtual memory
From the Virtual Memory window, uncheck the box for automatically managing your page file sizes, select your drive from the list and define a new custom size for the page file. Note that you can also move the page file to another drive from the same window...
  • Select the drive with the page file from the list
  • Click No paging file and then Set to disable the page file on this drive
  • Select another drive and choose Custom size or System managed size to enable the page file elsewhere
As for how large your page file should be, Microsoft's definitive guide on the subject says there's no value that will work for every machine. However, the company suggests that adding some performance counters to the Windows Performance Monitor (perfmon.exe) will provide information which can be used as a baseline to determine the best page file size for your system. We reduced our page file from 5360MB to 4000MB.

A quick highlight reel of our drive cleanup

To recap, we've included some of the ways that we regained the 27GB on our Windows drive. This isn't a cheat sheet for the article because there are more methods above for cleaning your drive than we actually used, but we thought a play-by-play of our clean-out might be useful to someone in a similar situation.

Here's most of what we did:

  • No longer use an Apple device, deleted everything Apple (including iTunes and Safari, mobile backups etc.) Had to manually delete 457MB worth of files at C:\Users\TechSpot\Music\iTunes and another 500MB of Apple data from C:\ProgramData. (+1.6GB storage space)
  • Uninstalled other old unused software from Windows' built-in Add or remove programs utility. (+3.9GB)
  • Manually deleting program files found via SpaceSniffer (Avira Scout for instance wasn't listed in add/remove programs along with other leftover files from uninstalled applications, not to mention a random VLC stream file and 710MB of files in an FlvtoConverter folder) (+many many gigs)
  • Deleting the contents of this folder recovered several more gigs worth of storage: C:\Users\TechSpot\AppData\Local\Temp
  • Scanned with Disk Cleanup -- basic and advanced (+200MB)
  • Scanned with Wise Disk Cleaner (+2.2GB)
  • Deleted the contents of C:\Program Files\Nvidia Corporation\Installer2 (+1.1GB)
  • Cleaned DriverStore folder with Driver Store Explorer (+2.42GB)
  • Cleaned WinSxS folder via Command Prompt (+1.58GB)
  • Compressed various folder around the operating system
  • Reduced page file from 5360MB to 4000MB (+1.3GB)
  • (Already had hibernation disabled and System Restore minimized)
  • Final scans with Wise + a system reboot resulted in 37GB of free drive space, up from 10.5GB at the start of cleaning.