I tryed this on my dad's laptop a couple of months back, and it cleared up half a gig.
concr3te just tryed it on his pc, and it freed up 6Gb!
As far as I know, there are two ways to do this:
1. If you run firefox: Go to tools -> clear private data. Or, if you want to schedule it to clear 'private data' (*cough*Santa*cough*) when you close firefox go to
Tools->options->privacy and check the boxes you want.
2. Download
This tool and use it to wipe out all the crap on your pc. Only problem I had with it was that I accidentaly deleted the passwords saved in my browser...which was actually quite a hinderence and I spent days making new e-mails etc..
Anyway, enjoy
