It is very likely your computer is infected with Spyware that normal Antivirus programs cannot Detect.
Spyware & Adware are files that can be installed on your computer, even if you don't want them, without you knowing they are there! They allow companies to monitor your Internet browsing patterns, see what you purchase and even allow companies to inundate you with those annoying "pop up" ads!
If you've downloaded some music, files or documents and suddenly started getting annoying ads popping up on your screen, you could definitely be infected with SpyWare and/or AdWare!
Spyware Remover will scan your PC and let you know if you have any files that are infecting your PC.