Did you do anything to get the money?? How do you know the company is legit? Call up the company but don't use the phone number on the check, look it up online or in the phone book. Also, sometimes you get checks in the mail, usually for much less, but by cashing them you automatically enroll in programs. Read any materials you got with the check because these programs can be killer, and can start automatically taking money out of your account without your actual consent.
It sounds fishy to me, if you've gotten the check out of the blue.