Lavasoft's Ad-aware program has been in the malware game since the beginning.  The basic still remains to be free and while it does not scan your background files it does do a fine job when you remember to run it.  The update does run in the background and continue to keep you up to date with the latest malware detections. 

I have been using this software for years and continue to recommend it to everyone. Its very easy to use and its free. The ones you pay for provide more advanced functions but you don't really need them. Check it out here:

I found this software awhile ago and haven't looked back. Not only is it free but it doesn't take up a lot of resources like the home verisons of mcafee or norton. I found that it protects just as good as those pieces of software without all the bloating fuss. Its made form the popular linux antivirus software called clamAV. It specific feature include scanning scheduler, automatic virus database updates, atandalone virus scanner, integration to MS Windows Explorer, and add-in to MS Outlook.

Check it out here