Which should be used wisely, as many times, it can delete a program's config file, and you'd have to configure it again.

There are two main tabs on the side. One is marked Cleaner which you want to use. It's the default. But like Kavazov mentions you want to check through the options to make sure it's not about to delete anything you might actually want.

The Registry option I'd only recommend using if you have any problems that might relate to the registry in some way. Speed isn't one of them.

If you're trying to speed up your machine, just a good old chkdsk /r from command line can help - especially if it cleans up more that 1000 indexing errors!
/r can take a LONG time though, maybe a chkdsk /f instead if you're confident your HD is physically fine...

Just to add... one of the annoying things about the Cleaner part is the fact that it can delete Microsoft Security Essentials' logs, and then the anti-virus will bother you that you've not run a scan in a long time (even though you ran it a few minutes ago!). The AV seems to depend on its logs to check when was the last time that you'd a scan.

(dunno if this was fixed in the last few versions, but the problem was present until one or two months ago (haven't used CCleaner since then to test this)).

As others have said CCleaner great utility and its free so even better for it :)

I can't say I've ever had that problem. So maybe there's a box you had checked which I haven't and it's deleting things it shouldn't as a result?

Either the Windows Defender or MS AntiMalware check-boxes were causing it, can't remember which one exactly.