I get these calls on a regular basis, I have a number of responses which depend on my mood and whether I have the time to play....
Put phone down when I hear it ringing other end (obvious autodialer picked up at other end only when answer detected).
"Stop calling me you scamming B*****d
[indian accent] Hello my name is trevor [/indian accent] I say "I bet it isn't"....this usually causes a short discussion to try to convince either way, followed by termination of call.
"Oh god which computer has the fault, I have 10 running at the moment" I go on to suggest that if he has detected a problem he must be able to tell me which one has the fault as I am really worried about this... can be extended to cover a full tea break!
The trouble is with this is that there must be enough people who fall for it or else they would not keep on doing it....