I would think an engineer at your ISP would by now have heard of the fake PC tech scam. See this thread:

Here's the verdict straight from Microsoft:

Avoid tech support phone scams

Strange call pretending to be from Microsoft tech support

Some relief came the first week in October 2012 as the FTC spanked whole gangs of these creeps with huge damage awards.

FTC Halts Massive Tech Support Scams
 Oct 18th, 2012
Caller said we were sending out spam and that someone would pay if it kept up... We're an ISP and do not send out spam.
Your local ISP
 Oct 18th, 2012