First of all, stay the hell away from vista. They are still woking out bugs,, and MANY software and games that were coming out at the same time as Vista's release, were NOT made to preform on Vista. And just because you have a Good system.. might not be "good enough" for Vista, read up on it.
(Just as good or even better you ask... ummm YEAH!)
Second, Free Antivirus!
YES, you are SMART, why?? McAfee and Norton are crap.. why? Do you know how much they would have to PAY testers on all different operating systems, different configurations etc?? Free Companies "we" are the testers so more things are caught with more testers!!
AVG makes a great one: grisoft.com
Spyware terminator: spywareterminator.com
spyware killer and clamwin antivirus.. not bad I like the terminator part.
kaspersky.com : people swear by this, I just haven't gotten around to using it yet... next one that fails me, I'm trying this.
Also other things to have on hand:
zonelabs.com for a firewall if you THINK you need one.. most these others have "real time" protection..
CCleaner.com (crap cleaner.. get leftover from programs and over flows etc .. OFF!)
Hijack this (trend micro systems bought it out) it's still works great and is good at many a problem !
Glarysoft.com good registry cleaner and repair.
PS. I keep 2 antivirus on mine at all time. One active and one sleeps... When I do my once a week thing, I scan with the normal, update and scan with the other, .... just in case!