Re: anti virus
I use AVG free edition. Avira and Avast are also good. AVG is good because it uses very little memory and is good for basic protection. For anything more than basic protection I'd say go with Avira or Avast. Avira has rootkit protection and avast has more ways of protecting than AVG does. You could try until you find which one you like, that's what I do especially since the versions are changing constantly.
Desktop: Dell Dimension E510, 2.5GB RAM, Intel P4 3.0GHZ, 250GB HDD, Windows Vista Premium. Might not be the best, but the old girl is still running good.