Well, I would suggest to decline the caller, and tell them they are mistaken. I would then tell them that you will take it to a local repair shop.
This actually happened to my inlaws! I suspect my father in-law was doing something he shouldn't have been and he got hijacked. He actually paid the $250, to get "Clean". $2500 later his pc was still hacked up. He went to his local police and they said no law had been broken (if a waitress were to charge you $1200 for a $90 dinner, she would have been arrested). So he went to his bank, and they stopped the transfer, giving his money back. The caller calls him daily, and actually locked up his PC. Once again, the caller has his number, and calls him all the time!
With the last statement being said, is why I would deny that you even have a PC and hang up! If they think it is a waste of time, they will go to the next one!
The bad thing is, he didn't ask me for help. So he paid for it!