If that didnt happen before, it would mean you got mal ware in your computer. If your browser is using pop blocker already (if not then you should activate it) then I will suggest you use a software for finding malware like Ad-Aware http://www.lavasoft.com/
They have a free version it might help you delete the malware. Basically that program goes through your computer and deletes all the junk that gets downloaded without you noticing while you surf
Also upgrade your antivirus and do a full system scan
Good luck
