Grisoft
Grisoft makes a free version of their av scanner.  I do not recommend it unless you are very careful.  This is becuase it only allows one automatic update per day.  In todays world, that's not frequent enough.  Why?  Because typically a home user will start their computer and immediately check their email.  And the virus scanner has not been updated yet.  That's the most common way to get a virus.  A home user needs an automated way to make sure that the email scanner is up-to-date.  My home network, main site work network, and two small business that I consult for all check each hour for new av updates.  Grisoft's product has a two year license for around $20 per year per computer.

Trend Micro
I like Trend's corporate products, but their consumer products need work.  I like Grisoft and Norton better.

Kaspersky
I used Kaspersky last in early 2001.  I liked the notification options.  The consumer product had lots of corporate features.  We moved to Norton when the 1 year of updates were over.

I have used free on-line virus scanners with great success, look at the Links (Security) section for a list.  We are starting to see online spy/adware scanners as well.

Needs further research
Free antivirus programs, I have not checked this out http://http://www.free-av.com/