It could be adware.
You may need to switch off System Restore as back up files may be hiding in there, and it will just restore itself. Go to Start - programs - accessories - system tools - system restore. Click the link on the left, follow the instructions.
Run Ad-Aware SE and AVG anti spyware in Safe Mode. To get into Safe Mode repeatedly press the F8 key after the bios screen loads but before Windows loads. You'll know if you made it because the screen will be at 800 x 600 and your pc will be slow.
You need all these anti spyware programs;
http://free.grisoft.com/doc/20/lng/us/tpl/v5 - AVG anti spyware
http://www.safer-networking.org - Spybot Search and Destroy anti spyware. Use the Teatimer settings to protect Internet Explorer, and the Immuniser to stop malware getting on your pc.
Spywareblaster works with Internet Explorer and Firefox. It won't uninstall spyware that is already on your pc, but it will prevent hijacks and other malicious changes to your browser.
http://www.javacoolsoftware.com
Plus;
http://www.lavasoft.com - Ad-Aware SE Personal anti adware
And these are useful utilities;
http://www.mozilla.com/ - Firefox web browser.
https://addons.mozilla.org/search.php?q=ad+block&type=A&app=firefox - Adblock Plus
http://www.ccleaner.com - Cr*pcleaner Registry cleaner and privacy tool
Winpatrol is a handy utility that will manage your startup programs and cookies.
http://www.winpatrol.com
http://www.siteadvisor.com -MacAffee Site Advisor warns about malware on sites you are about to visit.
http://spywarewarrior.com/rogue_anti-spyware.htm - The Spyware Warrior site identifies fake anti spyware programs.