Assuming you try out the suggestions for transposed letters and numbers above...
There are tons and tons and TONS of fake Windows 7 packages out there with either outright fake keys or keys that have by now been blacklisted. Mostly overseas but they do make it into the US and get sold at swapmeets and such.
You need to go back to the person that "gave it to you" to get it strightened out. They may not know it is fake, the person they bought it from might not know it's fake.
But any reputable dealer should take it back or exchange it, if it was bought from a guy that knows a guy and handed to you for free... you're probably looking at buying a legit copy in the future.