I'm still running Win98 SE in one of my computers without problems.
What anti-virus are you using? I'm running AVG, which is free and excellent.
Are you using Internet Explorer? Get rid of that Microsoft crap and move to Firefox. Much fewer bugs.
I also run two anti-spyware a couple times amonth in my computer. Spybot and Ad AdAware. Both free and rated very good. They can catch some stuff anti-virus programs do not.
Also, as a last security measure. Do not let websites install cookies and run scripts at will. Configure your web browser to ask you before. And in most cases say "NO". You would be scared by the number of spyware crap that bad websites try to inject in our computers.
I have Win XP Pro and I'm not impressed. I disabled all crappy "security measures" Microsoft (does it belong to a liberal crook?) and run my own firewall in my router, etc. Actually, I'm planning to move to Linux.
Good luck!
P.S. AR15 website so far has been harmless.
ETA: Damn... three weeks since I sent the money for my Team Membership. How long does it take to upgrade?