if your talking about a windows installation, i suggest Avira. its free, has one of the highest detection rates for AV, and it takes almost NO resources to run the active guard. (AV scans will muck your computer a small bit, but not much.) Ive been using avira in conjunction with Spbybot and SUPERantispyware for years, and ive only ever gotten one virus that couldnt be easily removed with one of those tooles.
