New major problem!

I downloaded Anti-rootkit and rebooted, and upon reloading, received the following window, looking very much like a Microsoft window:

Microsoft piracy control
Your copy of windows was activated by another user.

etc. etc. about billing details but not charging a credit card

Then two choices: Yes, activate Windows or No, I will do it Later

The first choice asks for credit card, ATM, etc. Obviously this is not Windows, but a hacker. The second choice shuts down my computer. I cannot do anything outside this window, and it also cannot be closed.

So now I have a new something -- this hasn't popped up before -- and I can't get into Normal Mode at all. Please advise! Can I run anti-rootkit in Safe Mode and otherwise proceed exactly as before?

Should I just reformat? I'm trying desperately to make that an absolute last resort.