ClamWin is one of only a handful of free antivirus tools for Windows, the
most popular of which is AVG Antivirus.
It's based on the Clam Antivirus engine which, like ClamWin, uses the GNU
General Public Licence making it open source software.
It doesn't have the slickest of interfaces but this isn't really a problem
as, once it's installed and you've set its options to suit your needs, you'll
rarely go into the application itself.
You can set the software to scan your PC at regular intervals, and it
includes context menu integration in Windows Explorer so you can scan a file or
folder simply by right-clicking and selecting 'scan'.
ClamWin can be set to automatically download updates to the virus
definitions, ensuring that you are as safe as possible. It also includes a
plug-in for Microsoft Outlook, should this be your email client.
On the downside, it doesn't have a real-time scanner so it won't
automatically check downloaded files for viruses. The Outlook plug-in, however,
will delete a virus-infected attachment immediately.