crap cleaner, ad-aware, and diskeeper are all good. crap cleaner cleans your comp, ad-aware cleans spy-ware, and diskeeper is a much faster and more effiecent defrag tool then the default winxp one. Well not too much better, but it's faster to defrag.
I agree with Barney's Soul. Also, try ewido security suite for the hard to catch viruses. It caught 50 while Norton Anti-Virus caught only 15 (mind you, ewido also gets mal-ware, but stick with Spybot and Ad-Aware).