These anti-virus scanners are free for personal use.  Never install more than one anti-virus scanner on a computer.  Uninstall your current anti-virus program before installing another.

Avast Home Edition, my personal favorite, is the program I have used longest.  Never had a problem with it. Avast requires periodic re-registration.  Otherwise, it is the least obtrusive free A/V program I have found.

AVG  Scheduled signature updates and scans tend to be somewhat disruptive when you're in the middle of a project.  I shut off the scan scheduler, and scan during regular weekly maintenance.  Daily signature updates are scheduled at supper time. A nice feature of this free scanner is that it allows system shut down on completion of scans.

Antivir gets pretty good reviews, so I installed it on a Windows XP sp2 machine.  Installation was quick and flawless.  The first scan, which included a 20GB main drive and a 60GB secondary drive, took about 35 minutes.  The user interface is uncluttered, and easy to use.  A daily popup nags about upgrading.  You must click OK to close it.

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All anti-virus programs need up-to-date information, referred to as "signatures" or "virus databases", in order to detect new viruses.  These signatures should be updated daily. Most anti-virus programs provide automatic updates.