Clamwin has been a trusted anti-virus solution for years. Clamwin stays resident, has scheduled scans, and integrates with Windows Explorer.
I tried the avast! solution on an older XP system.
The product installs a couple processes that start automatically as services, using about 25M or RAM. The avast! full boot scan is a great way to find and remove trojans. A few problems were found (in the recycle bin and in an old system restore point).
Results of full virus scan.
Due to RAM constraints of the older PC, I uninstalled the product after a boot-time trojan scan and full virus scan.