Cookie, it could be anything from a virus to some spyware/cookie that a website left behind. Do run a good virus check and anti-spyway check. You can google those if you don't already have them and at least get a 30-day trial. Use some reputable software though that doesn't use popups to advertise themselve...sort of ironic but a lot of less reputable anti-virus and anti-spyware folks do advtertise via popup. Popups are often the culprits of spreading spyware. McAfee and Norton are two that I would highly recommend. Also, run a firewall (McAfee and Norton also provide) and don't enable popups in your browser.

Another possible scenario though is that your computer is experiencing difficulties with the operating system (Windows) or some hardware conflict. That right there is a lot of troubleshooting and without knowing exactly what is happening (can't read your screen), I can't give you any more information beyond that. It might be worthwhile to find some reputable tech support person who can rebuild your Windows installation, do a defrag of your hard drive, and verify that your hardware is all working correctly.