"Junk file" is a marketing term, not a Windows category. Here is what actually sits behind it, sorted by how comfortable you should feel deleting it.
Safe to delete
- Temporary files — scratch space from installers and running programs; anything still in use is locked and skipped,
- Browser cache — rebuilds automatically as you browse,
- Thumbnail cache — regenerates the next time you open a picture folder,
- Recycle Bin — assuming you checked it,
- Crash dumps and error reports — useful only while diagnosing an active problem,
- Windows update leftovers — once you are confident the update is stable,
- Old log files — read by almost nobody after the week they were written.
Think before deleting
- Cookies — you will be signed out of everything,
- Saved form data — addresses and details you type often disappear with it,
- Downloads folder — technically clutter, frequently the only copy of something,
- Installer packages — re-downloading is fine on fast internet, painful on slow.
Never touch
- Anything inside C:\Windows\System32,
- Program Files folders for software you still use,
- The WinSxS folder — it looks enormous but is heavily deduplicated; use Disk Cleanup instead of File Explorer,
- Driver stores and restore points, unless space is genuinely critical.
A word about registry cleaners
Removing registry entries saves a negligible amount of space and occasionally breaks a working install. Modern Windows does not accumulate meaningful registry rot, and the performance claims made for these tools have never held up in testing.
Doing it with a tool
Every category in the safe list above maps to a cleaner in BleachBit, and Preview lets you inspect each one before committing. Start with the temporary file guide or the FAQ.