http://www.bitdefender.com/solutions/free.html
just became free!
pretty much the top rated virus detection for many years in a row, it isn't "that" resource friendly it is using 130MB of ram, to me that's nothing.. but it will catch pretty much everything
I rock immunet (clamAV on) with bit defender now
dropped down to 45MB total now, so it was maybe doing a little scan
I'd say with clamAV and bit defender there are no viruses that will get anywhere close to your system, malwarebytes of course is always nice to scan every month or so