As others have alluded to, you have spyware/adware/malware on your system. One of those components is almost certainly responsible for the pop-ups. Anti-virus programs don't detect these intruders, so you'll need to install utilities specifically designed for the job.
Ad Aware and SpyBot Search and Destroy are two excellent utilities for detecting and removing these types of offensive programs. Both are free to download and very easy to use. I highly suggest installing both programs and running them consecutively; one program will often catch something that the other missed.
Linkage:
Ad Aware: http://www.lavasoftusa.com/
SpyBot S&D: http://www.safer-networking.org/index.php