problem with your windows computer

I am curious as to how many people have had phone calls from some Indian sounding callers saying you are having problems with your windows computer.

Thumbs up please from any one that has had that call, and a reply please from anyone that knows what this scam is after?
I have played along with them for a while a few times but can't work out what they are after.

I must have had at least 30-40 calls from these guys over a period of at least 6-12 months.
How do I know its a scam?
First it sounds every bit like a scam, and second my father had them call him saying the same thing (including their internal feedback from his computer tells them he has a problem) and he has never even owned a computer.