Did you double check the number? From Microsoft:
1. Verify that you are typing the product key and that you are not
typing the CD-set number, the disc assembly number, or the part number.
The product key is located inside the disc case or on the back of the
disc sleeve on a rectangular sticker, and appears in the following format:
PRODUCT KEY: XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
Retype the product key. Make sure that you type it correctly.
2. Make sure that you type the correct characters. The following
characters are often confused:
. B and 8
. G and 6
. U and V
. Q and O
. The letter O and the number 0 (zero)
. The letter L and the number 1
3. Verify that you are using the product key with the correct product.
Product keys only work for the product that the product key is for.
