Claimed to be from Microsoft. I entertained the call for a little bit since I was a little bored. They asked you to go on to your console, go to to start up menu, and press a few buttons. I kept saying it wasn't working and they finally realized I was just messing with them. The guy said, "I know you are messing with us your MFer" and hung up. I take that as a small victory :)
Saame as above, calling from Microsoft regarding a problem on my computer. I am a Mac user and I told them I was reporting them to the FBI. He hung up of course.