For Windows, I also use AVG. Sockman, let AVG run in the system tray, it protects not just with email and scanning your drives, but dowloading and websites, as well. It uses almost NO resources, and does not slow your system down.
However, now that Mozilla Messenger has fixed the bug with saving all attachments, I won't use Lookout!! Express anymore. I won't use Internut Exploder unless I have to, which is extremely rare.
In fact, I have gotten used to getting my email in Mandrake Linux OS, which has the huge advantage of being totally immune to all those windows worms, VB Scripts, and viruses.
There are antivirus programs for Linux. However, those are mostly, if not all, for Servers, mail servers in particular.
Heuristics engines in antivirus apps don't totally depend on the definitions file for finding those pests.
The primary reason for AV apps for Linux, is to clean the windows pests out of email that is usually going through sendmail (which handles most of the internet email).