If you haven't opened the box, you should be able to return it, perhaps paying a restocking fee if you bought it online. If the package is opened, a return is doubtful, maybe depending on the employee you get a hold of. If you've used the key already, I don't think there's anything you can do.
When you buy Windows 7 it comes with 32 bits AND 64 bits. (2 DVD but only 1 key)
If you activate one version you cant activate the other one and vice versa. But the same key works either for 32 bits or 64 bits.