Virus or not? Will do a "quick scan of my computer". help?

okay.. so I was looking at some picture on google. (not porn lol) and then all of a sudden, bam. this thing pops up and says ehr.. something ad will do a quick scan of your computer.. it's still using my firefox, but it's like.. a box that only says ok, and i can't close it, and it won't go away when I click ok. I think it was adsense or adaware or some ****. anyways, i totally freaked, and i tried to close the page but it didn't work. And so.. i turned off my computer.. When i turned it back on, of course, firefox saved it, and it came back to that page. anywho, since it was still loading, i had time to close it this time... I think it was just an ad.. but I'm kinda freaked.. do you guys know what it is? I'm scared that whatever that was.. just scanned my computer, and like.. totally looked through everything. I don't have a lot of important things on it, but still. What if some creep totally collected my info, or is watching my every move? lol anyways, much help would be appreciated. thanks.