Do a malware check with some malware scanning programs on the Windows computer. You need to scan wit

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