You may have picked up a browser hijacker.
Do you have:
IE 5.01 with 1 security patch
IE 5.5 SP" with 1 security patch
or IE 6.1b with 2 or 3 security patches?
Do you have all Windows updates installed? (that doesn't mean the latest IE etc, just the security updates), do you have a hardware firewall, anti-virus or Spybot 1.3 (with updates)?
All of this stuff is needed to keep nasties out these days, even if you don't open unknown email attachments or go to iffy sites ;-)
Spybot is the thing for removing browser hijackers BTW, it can be downloaded from
http://safer-networking.org
