yeah there is some spyware/adware crap that will make your computer do that, and it keeps downloading it...its not an actual website popup it just looks like one.
Get "Ad-aware" free off several places ...just search
and also "HijackThis". Run Highjackthis and look and delete anything with winfixer in the the name..then run ad-aware...
you may have to repeat or switch up order...but once you get rid of it it wont come back.