Ken:
It seems that you could infected with a specific brand of bug called a browser hijacker. Anti-virus software won't find and delete them. Unfortunately regular sypware might not be as effective. There are many variantes of these bugs so I won't even go into details which one could be.
I've successfully treated some infected computers such as yours following these simple outlined steps.
Try downloading and running two freebie applications. First one is CWShredder that might solve your problem.
You can download it from here:
http://www.intermute.com/spysubtract/cwshredder_download.html
if you are still having the problem try the second one Hijackthis.
http://www.merijn.org/files/hijackthis.zip
Also, this tools are for removing the bug only. To keep yourself free from this pests you will nee to.
TO keep from being re-infected. You must do this:
1-) Keep your antivirus subscription renewed and the liveupdate running on a regular basis.
2-) Keep a Firewall I recommend Zone alarm it is very good and you can get a trial version for free.
3-)Use the free sypware tools like Microsoft defender beta version and Yahoo tool bar with anti-spyware.
4-) you might also try Ad-aware 6.0 it's also free and very effective.
Good luck and keep me posted.
Cesar
MCSE, MCP, A+, WebDesigner