I've had a number of calls like this, I usually just hang up, but one really tried to pretend they were official, so I strung along for a few minutes.
He said he was from Windows, so I then said:
"Ahh... you must be calling from Microsoft then?"
"No Sir, I'm officially from Windows, the operating system on your computer".
"Oh, so you aren't Microsoft then...hmm".
"No Sir, we are Windows".
"Do you know who makes Windows?"
*Umms and Arrs a bit*
"I believe Windows is made by Microsoft..."
Yes, that's right Sir".
"So you are calling from Windows, which is owned by Microsoft, but you aren't Microsoft?"
*Umms and Arrs some more... then talks to somebody in the background*
"Listen Sir, I'm calling from Windows, there is a problem with your computer".
"Right, I'm with you now, that does sound quite bad, actually I'm quite worried, what should I do? Are you going to be able to help me?........ Is it going to be a problem that I'm not using Windows? My computer is made by Apple. What about my Xbox, that's made by Microsoft though...?
*Hangs up*
They must get some people though, or else they wouldn't be making so many damn calls. I'm sure it's the same company who once phoned me from the "BT Carphone Warehouse" trying to give me a free phone.