Antivirus
Get a mac.  If you have convinced yourself you need a pc, then 
get a macbook or intel powered mac.  Since they can run XP now, it is pretty much the only computer you will ever need.
If you refuse to get one, or already have a nasty virus on your pc 
(macs don't get viruses) you need PC Cillin by Trend Micro  (do not waste your time with other programs)