Some users find bloatware like McAfee or Norton work well.
Seems you're not one of them.
The "big three" freeware AV's are
Avast!,
AVG, and
Avira. (I use Avast, find it problem free in all respects.) They're all pretty well respected and offer good protection. Avast is maybe the most configurable, and the engine/detections in the free version is identical to the paid one.
Be sure to run the
removal tool after uninstalling McAfee from the control panel.
Check that you have a firewall running after removing it, too. Vista has a 2 way firewall built in, XP's is effective, but only blocks inbound. There are a few good free third party firewalls around, and at least 4 good free antispyware applications if you need some of them, also.