Did you enter this "lottery"?
In all probability they are hoping you are a sucker powered by greed. The letter will probably ask you to send in your bank account information to allegedly electronic transfer the money. What they will do is clean out your account. DO NOT send them this info and DO NOT send any money. DO NOT deposit any "check" they send you. They can get your checking account information that way and leave you holding the bag.
However, the big question is did you enter this so-called lottery? Probably not. So, if you didn't enter, how can you win???