Hello Jim.
There's no best free anti-virus program, but the three promiment ones are listed in the link Cart posted above.
SUPERAntiSpyware is a solid anti-spyware program, and I've found it to pick up things that Ad-aware,Spybot S&D, and AVG-AS don't detect. All these are on-demand scanners, and shouldn't take up much memory (unless you run all of them together for some weird reason!)
Let me know if you need any more information.
