New spin on the fake Microsoft calls. Usually the scammers call you but these guys also put effort into getting high on Google's rankings and people searching for "microsoft tech support" will end up calling them. Sad but true.

Either way, they'll get you to let them remote into your PC and try to get your credit card info. The new spin is that, once someone has said "No", they will run a specific command on the PC that will lock up a very important file needed to run Windows, effectively locking your machine down. A last second punishment for not paying them.
Bottom-line: If someone calls you out of the blue, don't give them access to your PC. EVER. If you call them, well, make sure you got the phone number off of a page that belongs (and probably includes) "microsoft.com" (and not "somethingelse-microsoft.com")