Disk cleanup tools show you a number and a button. The number is the easy part. Here is what is behind each category, and the ones where the right answer is to leave them alone.
Safe, and you lose nothing
These are files Windows and your programs would recreate anyway. Deleting them costs you nothing but the seconds it takes to rebuild a cache.
This is where most of the space usually is, and it is also where most cleaners stop explaining.
Windows Update leftovers: safe, with one condition
After an update installs, Windows keeps the old files so you can uninstall that update. Once you are sure the machine is fine — a few days of normal use — these are dead weight, and on a machine that has had a feature update they can be many gigabytes.
The condition is exactly that: if you delete them, you can no longer uninstall those updates. If a monitor has been flickering since Tuesday, deal with that first.
WinSxS: do not delete it, and do not trust anyone who says you can
The WinSxS folder looks enormous in Explorer and it is one of the most common pieces of bad advice on the internet. Most of what you see there is not really there — Windows uses hard links, so the same file is counted many times over. Deleting from that folder by hand breaks Windows in ways that are hard to repair.
There is a supported way to shrink it, and only one: the component cleanup that Windows itself performs, which removes superseded component versions and nothing else.
Where the honest answer is: keep them
System restore points are the most valuable thing on a Windows disk that is not your own files. They are the difference between a bad driver being a twenty-minute annoyance and a lost evening. A cleaner that offers to delete them to show you a bigger number is optimising for the number, not for you.
The hibernation file is the other one people get wrong. Yes, deleting it frees several gigabytes. It also turns off hibernation and, on most laptops, fast startup with it. If you use either, that is a bad trade for disk space you were not short of.
FAQ
How much space will I actually get back?
On a machine that has never been cleaned, typically a few hundred megabytes to a few gigabytes, most of it browser cache and update leftovers. Anyone promising tens of gigabytes is counting things you should keep.
Does cleaning make my PC faster?
Almost never, and it is worth being blunt about it. Free disk space helps when the drive is nearly full. Otherwise you are reclaiming space, not speed.
Is the registry worth cleaning?
No. Registry cleaners have been solving a problem that stopped mattering fifteen years ago, and a wrong deletion there is a real risk. Rain Cleaner does not touch the registry for cleaning.
If you prefer seeing cleanup candidates before removing them, Rain Cleaner follows this approach with a preview step. I am one of the people behind it, so take that as a disclosure, not a recommendation: everything above works with Windows alone.
Originally published at https://raincleaner.eu/guides/what-is-safe-to-delete-windows
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