lost product key

hi my name is robert knowles. and i i am hopping you can help me. i have win 2000 professional on my computer and i want to put 98 on it instead .
i have the disk but the problem is i don’t have the product # i lost it and have no idiea hoe to get . if you can tell me how or any help you can offer that would be great

sencerly Robert R knowles
email address robrknowles@yahoo.ca