http://www.bitdefender.com
You can get either a free version or a full evaluation version.
bitdefender is THE best AND the least intrusive (and there's a free version on their site that's fully functional). When i say the best i mean in objective tests of which virus types it can recognize even without a specific definition for it compared to mcaffee or norton (ie finding new crap and fixing it before it makes the news).
AVG is totally free, but its not as good.