Normally, I'd expect to see an Invalid key message when you enter the key and it has been mis-typed, but it is certainly possible that a typo could get you a valid key.
If you have confirmed that they key you entered matches the label exactly, either contact support to confirm that this is a duplicate key or simply exchange the retail copy at Best Buy.
I'll add that if you did install and activate on another PC, your second and 3rd installs would need to be activated with the same LiveID, not they key and different LiveID.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve