There is every likelyhood you have spyware/adware on your hard drive. These will cause those popups as well as make your computer sluggish.
The first step towards correcting the problem is to download, install, and update both Ad-AwareSE as well as Spybot Search and Destroy, both free applications designed to find and eliminate this kind of malware.
For a more thorough discussion, and links to these two applications, you may read the following Tutorial with profit:http://www.bleepingcomputer.com/tutorials/understanding-spyware-browser-hijackers-and-dialers/
Post back if you still have problems after using these, and we can take further action.