Incorrect Product Key

Hello,

I have just purchased a new computer. A Windows 7 Acer, actually... Well, as a new computer, a Product Key is required... right? See, Microsoft Office is really important! I need it for my papers and powerpoint presentations and all those. I was prompted for the key... I found it on the sticker of the Certificate of Authenticity. But, after 3 times of making sure that I wrote down the correct characters and actually inputting them, it says it's incorrect! Why might it be incorrect? How? How can I fix this??