This goes by another name too (I think) called MacDefender. Believe me, it DID NOT come from this site. What you're seeing is harmless unless you gave them your CC number. It's easy to rid yourself of. I'm guessing you have a Mac, correct? If you need instructions let me know.
Edit:
To make sure I'm not out of line- Have you previously been aware of a program on your (guessing) mac called MacKeeper?
If no and you're on a Mac, what do you see if in your 'Download' folder using your viewer?
If you login as a different user on your Mac and visit this site, what happens?
If you're not on a Mac I apologize for my posts.