Grisoft AVG has a free personal antivirus solution. According to their website, you can use it without any limitations for the life of the product. On the other hand, they offer the following notices on the download page:

AVG’s world-class professional, technical product support is only available to registered customers of commercial AVG versions.

AVG Anti-Virus Free Edition cannot be installed on server operating systems (such as Windows Server 2003), nor can it be used for the scanning of network drives.

AVG Free Edition has the following restrictions:

• No customization — Scheduling options in the AVG Anti-Virus Free Edition are very limited (only one scheduled update per day, one scheduled scan per day etc.). Also the Resident Shield configuration is limited with no exclusion settings. AVG Anti-Virus Free Edition does not offer advanced testing options, password-protected archives reporting, adjustment of scan process priority and many other features.
• Slower updates — AVG Anti-Virus Free Edition receives updates via a lower priority service. Priority updating via high-speed servers is only available for commercial versions.
• No technical support!

There are a couple of things that AVG does that help to encourage you to buy their full version. Most notably, they add a 'signature' to every outgoing e-mail indicating that the e-mail has been scanned with AVG. This signature has a link to their site with the hope that people will visit and decide to purchase their product. A lot of e-mail programs add a signature to your e-mails (like MSN and Yahoo), so this is not as uncommon as you may think.

The signature can be disabled by going to Control Center | Email Scanner | Properties | Configure | Uncheck "Certify Mail" (Thanks to Richard for that tip!)

I have used AVG Free Edition before, and found little to complain about, and it might be just what you need!