I have been getting a lot of x rated pop ups even when I have no browser window open. I go away from my PC & when I come back theres an explicit picture waiting for me! I'm not a prude but I do have a six year old daughter who does not need to see things like that!!
You need to scan for adware/spyware with something like Adaware or a2 or SpybotS$D.You have almost certainly got at least one nasty on your system.I've seen this before connected with a browser hijacker called Xupiter, and they are not messenger popups.