im not sure sorry. cannot find much info for spot on search engine.
when does it appear? is this when visiting one website in particular etc?
its very normal to get pop ups like this, a lot of websites use them for funding. i personaly dont mind them too much but you could try a pop up ad stopper or a browser with one built in maybe.
as a precautionary measure i would suggest you run a check for spyware on your system with adaware or spybot or both and delete anything they find just in case click here click here update them before running.
if you have windows xp and just in case it could be the messenger service annoying you click here