What is safe to delete on your C: drive

Safe to clear

These build back up on their own and removing them costs you nothing but a slightly slower next launch:

• Temporary files. Your per-user temp folder (%TEMP%) and the system temp folder fill with installer scraps and half-written files. More on temp files here.
• Browser caches. Chrome, Edge, Brave, Vivaldi and Firefox each keep a cache that is often over a gigabyte. Clearing it only means pages reload once. Cookies, history and passwords are separate and should be left alone.
• Thumbnail and icon cache. Explorer rebuilds these automatically.
• Windows Update cache. Update packages that already installed sit in SoftwareDistribution doing nothing.
• The winget download cache. Installer files winget downloaded and never cleaned up. Details here.
• Crash dumps and error reports. Per-user crash dumps and Windows Error Reporting queues are only useful while you are actively debugging a crash.
• Old logs. Archived event logs and OneDrive sync logs.

Safe, but think first

• Windows.old. After a feature update Windows keeps your previous install for rollback, often 15 to 30 GB. Safe to remove once you are sure the new version is stable, but it is your only easy path back.
• Old installers in Downloads. The .exe and .msi files for apps you already installed. Safe to delete, but double-check you do not need the offline installer again.
• The Recycle Bin. Obvious, but worth saying: emptying it is the last step, not the first.

Do not touch

The fast, safe way to do all of it

Rather than hunt through each folder, run one read-only scan. Sifty's checkup looks across everything without changing a thing:

PS C:\> sifty checkup

To see reclaimable junk broken down by category, still without deleting:

PS C:\> sifty junk scan
  User temp files       537 MB
  Browser caches        1.2 GB
  Thumbnail cache       13 MB
  OneDrive sync logs    83 MB
Total reclaimable: 1.8 GB

Sifty covers eleven junk categories out of the box (plus Windows.old and old installers as opt-in extras), and it knows the protected folders above are off-limits. When you want to actually clean, add --apply:

PS C:\> sifty junk clean --apply   # asks first, sends to Recycle Bin

Everything goes to the Recycle Bin, and sifty undo reverses the last clean. To find what is actually eating the space beyond junk, point the analyzer at a folder:

PS C:\> sifty disk analyze C:\Users\you   # biggest folders and files