Suspect malware.
Update your anti-virus app and then run a full-system virus scan.

In addition, install Ad-Aware 6 free edition, start it, click its 'Check for Updates' link in the app to install updates, then use it to scan your system, and remove what it finds.
Ad-Aware:
http://www.lavasoftusa.com/support/download/

Download and run Hijack This, but don't remove anything with it yet:
http://www.merijn.org/files/hijackthis.zip
Follow the instructions at this Tutorial:
http://www.tomcoyote.org/hjt/
to save the log files and paste them into a new post at one of these forums:
http://forums.spywareinfo.com/index.php?showforum=11
http://www.net-integration.net/cgi-bin/forum/ikonboard.cgi
http://tomcoyote.org/forums/index.php?act=ST&f=4&tI9