[quote name='Sdawg']You can also check the full headers of the message to see if it actually originates from bestbuy.com. If they do need an affidavit, I'm pretty sure that they would send it to you via snail mail rather than an email.[/QUOTE]
Not entirely true. They may have you print it out, I had too, though, I was on strict time constraints. However, as he suggested (and I did above) check the headers and see who the email comes from. What you will find is that this stuff is not handled by Best Buy, but a partner who does this thing for multiple clients. Google the name of the company and you are bound to find something. I had my doubts, too, but I did a search and came across a press release that said this company was handling GameStop as a new client.
Again, this is assuming you won the grand prize. If you just won a game they shouldn't need all this and should just send your game.