The Better Business Bureau warns consumers not to fall for this. But if if this happens to you, immediately disconnect the computer from its power source--including the battery if you're working on a laptop.
There are several articles on preventing the threat of ransomeware, but here are 3 from Norton, Wired, and Carbonite.
Protect yourself, but NEVER pay the ransom! If you are unsure what to do, see the help of a professional, two we recommend locally are Premier Network Solutions or SuperHiTech.