ClamWin is a free antivirus tool that could be the ideal choice to add a second layer of protection to your system.
The program doesn't offer real-time protection, so isn't suitable for use as your main antivirus package. However this does mean you can install it alongside most other security tools without any problems, then use it to run scheduled or on-demand scans to get a second opinion on your PCs health.
ClamWin is also compatible with a wide range of Windows versions - 98, Me, 2000, XP, Vista, 7, Server 2003 and 2008 - so you'll be able to run it on just about any PC, even ancient systems that the other antivirus vendors ignore.
The program isn't overflowing with features, as the authors have just concentrated on the bare essentials: automatic downloads of its virus definition updates, a scanning scheduler, Explorer integration to help you scan specific files or folders, an Outlook add-in to remove dangerous attachments, and more.
Still, that makes it easy to use. If you think you've been infected by something that your main antivirus package has missed, then it only takes a moment to launch ClamWin, run a memory scan and see if it can find anything.
Note that this is the portable version of ClamWin.