I had to reinstall my Word 2002 today...after I had clean installed XP.
I typed the product code in, checked for accuracy, clicked next or OK, and it was invalid. I called Microsoft. They said it was valid and mentioned that it could have been because I had typed some blocks of the code in with the caps lock key depressed. The technician also noted that having your A-V software running (which I did) could also cause product codes to be rejected. He said it did not have to in capitals (as it always is when you get it on a CD or a CD case or container.)
So I tried again using all lower case letters (and a few #s.) It worked.
The technician also said that if necessary he could generate me a new product code. Neat!