You probably don't have a virus, but you do have some malware that's giving you the popup trying to get you to download and buy their product.
Uninstall all those free web browser add-ons
Uninstall all the extraneous programs you got for free like Limewire & such
Delete all your temporary internet files & cookies
Edit your startup files and delete what you don't need
Run disk cleanup
Download Avira Anti-virus
Download Spybot Search & Destroy
Download Ad-Aware SE personal
Download ZoneAlarm personal edition free firewall
Reboot and uninstall all your anit-virus stuff, all your spyware stuff, and all your malware stuff. De-activate your Windows Firewall
Reboot and install ZoneAlarm, then install Avira, Spybot, and Ad-Aware. Update and scan your computer one at a time.
Run disk cleanup again
Run disk defrag.
Good Luck