Whenever I try to start up my game, a message appears and it wont let me play the game.

"The product appears to be validated but the machine SID doesn't match. Try deleting the file C and re-activating"

I can't figure out what to do, I looked for a file named "C" but couldn't find anything, I also tried uninstalling and re-installing the game, but the message still pops up. I have had this game for several months now and this message has never appeared before, can anyone help me?

Thanks