Yo'...2 days ago I had that same shit happening to me! It's fuckin' gay...but it was only happening when I was posting lyrics to some songs off of "OHHLA.COM"...hasn't been doing it anymore, though...
its a program that was installed on your computer. you probably accidentally installed it when you were installing a totally different program.
1. Go to START
2. Then go to SETTINGS
3. Then go to CONTROL PANEL
4. Then go to ADD/REMOVE PROGRAMS
now you should see a list of all the programs on your computer. look for something that looks like some kind of corny advertising program. i had this same thing happen when i installed a program (i think i was installing kazaa...). and the program had an alligator icon.
if you can't find it then do like the other dude said and get you a spyware program.