Remember that wind-up...
...where the recipient pretended to be a police officer investigating a homicide detective and just about made the cold-caller crap themselves?
Imagine the fun:
(Phone rings) "Uh, hello?"
"Yes Sir/Madam. This is Bob from MS Security, your PC has a virus. We need to install anti-virus."
"Really. Wow. How do you know?"
"We monitored your PC and saw virus activity."
"OK. Please don't hang up, I just need to go into the other room."
"Sure." (Now find something that makes a clicking noise. Click it)
"Can you just repeat, you monitored my PC?"
"Oh yes, it's all part of the service and for a mere £75 we can cure you of virus!"
"Thank you for confirming on record that you monitored our systems."
"You have called and just confessed to hacking and monitoring a Royal Signals Networking Bunker" (or something)
"No point in hanging up, we have already traced this call and your location. Your co-operation may mitigate any action taken against you, which could include life in prison. You've got no defence, I have your confession on tape. So, tell me your real name and who you work for and I'll do my best to help you." (and just carry on making crap up for as long as they stay on the line or you can be bothered for)
You probably won't get anything from them...but it would at least provide some amusement.