CCleaner is truely the best method/program for cleaning up your PC, apart from formatting the hard drive of course
I run a scan daily, with all the options/boxes ticked, I usually get around 300MB of space cleaned up after running this, that shows how much I do on my PC per day, I spend 7 hours per day (Monday-friday) on my laptop, so you can understand how the rubbish builds up.
Give it a try if you're not doing so already.
http://download.cnet.com/ccleaner/