After running CCleaner's registry cleaner, it shows me a lot (~100) of uninstall errors for Windows updates, like KB901214, KB917422,...

is it safe to delete these entries or will I crash my system on the next boot doing this?

Well a lot of these Windows updates will be listed in Add/Remove programs, check in there for the KB numbers first. Smilie copies from a previous post of mine.

Quote button not working, No simlies or upload button showing again on my works WinXP.

Why does CCleaner list these updates if there aren't any errors with them?

They are only listed because they take up space not because they have errors.

Its in the Beginner's Guide under Hotfix Uninstallers:
http://www.ccleanerbeginnersguide.com <== contains a wealth of information

If you delete the Hotfix uninstallers CCleaner doesn't bother to also remove the uninstall information. Therefore just about any registry cleaner including the one in CCleaner will find invalid uninstall information.

Note that CCleaner's registry cleaner doesn't find all of the invalid data, you could use RegSeeker to find the rest but be very careful using RegSeeker.

With so many Hot fixes coming out ,I would not use the Hot Uninstall cleaning option.
You will not be able to uninstall a Hot fix that does not work for you.
I have not had to uninstall a Hot fix yet but I have only been applying Hot fixes for the past year.
Others may have more information about the need to uninstall a Hot fix.
Best wishes, davey

Technically, now that CCleaner ignores the $hf_mig$ folder you can still uninstall hotfixes after the uninstaller has been deleted. However, it is more work once the Uninstaller has been deleted, and unless you're an advanced user I would not suggest removing those folders.

Also it probably takes a year or longer just to accumulate enough of them.