Very simple answer. Use one computer for Yahoo Messenger and all the other stuff for fun but that you could really care less about. Then, if you get the virus, swamp in a new hard drive and start over.
Never under any circumstances use YM on a computer with sensitive information. For that matter, do not browse the net with the same computer. Have two computeres. They are cheap enough these days.
I spend most of time on the internet using win 98. Works ok and not many attacks.
I recently got a worm on my 530n HP which is a 2.8mhz and runs winxp home edition. It was, was disabling and rendering the virus protection, and malware programs useless. Damn, they are getting sophisticated. It eventually locked up the computer, or I killed some crutial function, trying to get rid of it. All I could get was a blank screen.
HP has the partitioned hard drive with restore capability....you don't have to reload the operating system and drivers from disks. Never used it before but it restored everything....amazing.