"Messenger Service?"

[BCWWF]

What the hell is this? I just updated my computer to WinXP (If that matters) and now when I am online, like every 10 minutes or so some random pop-up comes up titles "Messenger Service" and it has random super generic ads. It is just a grey screen with black writing and a button that says ok. I dont know what the point of it even is, you are supposed to write down the web site, press ok, and then retype the website into your browser. Its not like a web page, and there is no information at all, I have no idea what it is or how I got it. I ran Norton but its still coming up. HELP ME!!!