the problem isnt with the virus but with the malwares and freewares ( and windows security holes ( which I doubt if your computer is running on local ip which is behind a router ) )
so basically all you need to take care of your machine is NOT to go to website with lots of those russian banners ( free porn websites mostly ) and prolly install a lavasoft ad-aware professional SE or personal one and scan regularly
1 more point : I do not see how virus would get into your computer especailly in this era ( maybe 1990s when DOS was popular
)
and also I do not recommend you installing an ANTI Virus but more like patching your windows through windows update regularly ( the critical ones ) because that would slow down your computer for nothing
why don't you install anti virus ? for example some weird russian guy made this virus called : ilovefishing or blabla and then this security thing appears in a computer security discussion and it would take another week for norton antivirus to get that virus software updated while your system ALREADY CRASHED if you get my point........
I personally do not use any setups and my system have been steady for 2 years without being reinstall and those craps ( windows xp professional SP 2 )
if you compare windows xp professional and windows xp home
please pick windows xp professional even its more expensive but will get your system faster and not installing those unnecessary programs that you will never use anyway...