Sometimes you will get popups from the sites people snagged pictures from on threads. I used to get that occasionally before I blocked popups.
Either that, or you've got Gator. Gater is an adware device that is a pain that way. Go to Start -- Control Panel and look through your ad/remove programs list and see if you see anything by Gator Corp.
Just ad aware which is strange because that is one of the pop ups I'm seeing this morning. My son was on the computer while I was out of town. No telling what he opened.