Unfortunately even the most intelligent people can get caught out on this. As former tech support, I try to warn my customers about this... But even then it may not be enough until they get done in the first time. One of my customers now had to lock their Web Bank and credit card because they took 10,000 from his account, and would have taken the whole near 80,000 over time if he hadn't noticed and put a hold on it.

Remember: Microsoft and your ISP will never call you and offer to fix your computer. Even at worst of times this is a paid service you should never allow anyone who is calling you to do. This article is dead on about how to protect yourself.

Put the proof of identity on them! They called you, they should be able to tell you your reference number and your IP Address at least. When in doubt, ask them for a call back number. Hang up, and do a search for that number. Call your provider directly and ask if it from them. At least then you know your talking to the real deal.