Sorry. Couldn't resist.
I just see soooooo many posts about how people lost CD key's and cerials for they're "legal" copy of a program. If I spent the kind of money that AutoCad costs, you better bet I wouldn't loose it.
I suggest if it's a legal copy, call the manufacture and fax them a receipt or something. They'll get you a new one.
If it's not a legal copy, don't ask.