The Inq published a guide with some advice on how to give an old PC a cleanup:
OVER TIME AND WITH USE, desktop computers gradually fill up with crap. It's a fact of life. The best way of dealing with it is to regularly back up your data and periodically wipe the hard disk and reinstall the OS and apps, then put your data back. The snag is, this is such a big job that nobody ever wants to do it, so it only gets done when you have to - if the hard disk dies or you get infested with malware or something.
But there are a couple of fairly quick and easy steps you can take you get some of the junk off a Windows box. Forget Microsoft's cleanup wizards or the many third-party tools - run 'em if you like, but they're so cautious they tend not to do much good.