You could have inadvertently installed some sort of toolbar, that will need identifying and removing, or it could be some malware problem, have a look at

• searches are redirected to another site
  after you read that try to explain what searches are redirecting, and if possible follow the advice it contains
• is malware the cause of my firefox problem
  the fact the problem is on both IE and Firefox could point to it being related to your ISP or malware external to the browsers.
• Cannot remove an add-on (extension or theme)

Post back with how you get on.

Do a malware check with some malware scanning programs on the Windows computer.
You need to scan with all programs because each program detects different malware.
Make sure that you update each program to get the latest version of their databases before doing a scan.

• http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware
• http://www.superantispyware.com/ - SuperAntispyware
• http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page
• http://www.safer-networking.org/en/index.html - Spybot Search & Destroy
• http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked