Hi Justin,
#1. Make really really sure that you are reading the product key correctly.
A little smudge, a scratch, can cause a 6 to look like a 5, or 8 look like a
9 or a 3, etc. Use a magnifying glass with a strong lamp, if available.
#2. Contact the dealer/place where you purchased the computer and explain
what is happening. If you are getting the product key number right - then
they may have messed up when they were installing. You may have been given
the wrong disk - they may used a slightly different version disk to set up
your computer.
Good luck, I know how frustrating this kind of thing can be.
Hope this helps,
Gordon