HELP!!! My computer is saying I have a worm in it and its trying to access my credit card information!!!?

I get this page that says I have the Lsas.Blaster.Keyloger worm. It says to activate the system security. It says it cost 50 dollars or 80 dollars for a lifetime. But it asks for my credit card information. If I enter my credit card information will someone get it. What should I do????????