You cannot install/reinstall without a 25-digit product key. See
http://support.microsoft.com/?kbid=823570 "How to obtain a new
product key for Office program setup". The 20-digit number you
have is the product ID mentioned in the referenced article.

Jim May wrote:

Just had Win XP installed (My MSO-Pro-XP didn't make it into the new
system - HD format performed). I have my install disc (copy) but not the
25-digit code required in step 1 of the install. Am I dead in the water? I
have a record of the code numbers that appeared in the install before -
Help - abount Excel, word and access - it shows only 20 characters; 5 short
of what's required to re-install...Again, am I dead in the water?/ Is there
anything I can do to reinstall Office Pro XP (with what I have)?