You acquired a virus, probably from clicking on some ad or otherwise shady link online. This same virus is trying to get you to pay the people who made the virus $200 dollars. Yahoo is likely not involved at all.

You should, as others have suggested, reformat your hard drive or if possible simply revert to an older backup. Instructions for these things can be found easily online.

In the future, avoid clicking links asking you to download something, and get yourself a decent antivirus such as avast or Microsoft Security Essentials.