Curiously, Microsoft have finally come out with what seems to be an effective product - Microsoft Security Essentials - which is free to download from their site. It updates onsight of a connection and scans according to your preferences selected from the Settings menu, defaulting to 02:00 on a Sunday. Change that to a daytime in your area, alter quick scan to full scan and restrict it to thirty percent of system resources and you hardly notice it scanning. As a resident, it notifies you of anything it wants to clean
In conjunction with Windows firewall, it's far less of a resource hog than McAfee, Norton and many of the others. Try it for a while - I think you'll agree. I thoroughly tested it for three months before installing it on customers computers and have had no complaints so far.