First ask yourself *why* you think you need an antivirus program of any sort, and then think if there are better ways to attack the root of the problem. Rather than running your PC in a straight jacket re-checking every file that is every opened, consider first if there are ways to stop malware getting on to your PC in the first place.
Never use MS email software (OE or O), use IE as little as possible (and lock it down to "high" security), make sure your incoming email is scanned (preferably at your ISP, or your own email gateway) and all executable attachments are removed, use a real firewall between your PC and untrusted computers (that may well include other family members, not just the Internet), check any dodgy downloads carefully, and most importantly of all - use your head!
For on-demand scanning of downloads (and scanning of emails if your ISP does not do it, or you don't trust them), try clam
On-demand virus scanners and complicated windows firewalls are an amazing waste of time, money, and PC resources, and they are always playing catch-up. A few proper precautions gives you better security for nothing.
(Of course, you could always switch to Linux...)