I just use two free ones. Sure they are free but they work well, and in practice usually better. Particularly with novice computer users who sometimes don't realize that when the Norton subscription runs out and the virus definitions database is out of date they are effectively no longer protected. I see this all the time, it is amazing.
Here is AVG Free (a free virus scanner that does auto updates):
http://www.grisoft.com/us/us_dwnl_free.php
If anyone uses it and still uses Outlook express, make sure to check the box for "use Outlook Express 5.0 Plugin" under the email scanner tab within the control panel. It works with Outlook express 6.x too. Also make sure at least all the "check incoming messages boxes" are set. Works very well.
Another free one is avast! :
http://www.avast.com/
It includes checking ICQ messages and such. I have not use it long but it appears to work well.
You might consider just using these free ones and then even paying for the professional versions if you like them.