Step 1. Delete restore points and turn off saving restore point
Click Start → Setting → About → System Protection
Tick: Disable system protection
At Max Usage, drag the slider to position 0. Click Delete to delete all restore points for this drive. Select Yes/Apply/OK
Step 2. Turn off the Reserved storage feature
Open CMD (Run as administrator), and type the command:
DISM /Online /Set-ReservedStorageState /State:Disabled
Enter, and wait...
Step 3. Clean up temp folder
Open Run, type: %temp%
Enter, then delete all files in this folder (Shift + Delete)
Step 4. Delete all file in Download folder
Open: C:\Windows\SoftwareDistribution\Download
Delete all files in this folder (Shift + Delete)
Step 5. Clean up system update files
Open CMD (Run as administrator), and type the command:
Dism.exe /online /Cleanup-Image /StartComponentCleanup
Enter, and wait...
Step 6. Empty Recycle Bin
Right click Recycle Bin on Desktop, select Empty Recycle Bin → Yes
After completing the above 6 steps, restart your computer. Now the C drive will definitely return to its real capacity.