Just put the phone down!

MICROSOFT TELEPHONE SCAM
This has been doing the rounds for quite some time now but it’s a problem still worth highlighting. You’ll get a call from an engineer who claims to work for Microsoft and reports that your system is badly infected with viruses. It’s at this point you should put the phone down! To rectify the issue (which doesn’t exist), they’ll for your credit card details and take control of your system. At this point, they have a free reign to do whatever they want on your computer.

Best advice I can give is:
1) To hang up immediately.

OR

2) Say that you used to be an IT engineer and you’re very interested to know how they can tell that your system is infected – won’t be long before they hang up!