validity of windows xp product key

i need to install MS visual C# 2005 express edition software to do a project. wen i put in the cd i get a message saying i need to install windows xp service pack 2. i hav downloaded it but during installation a message comes up saying ‘windows os product kay used to install os may not be valid’ and tells me to go to www.howtotell.com where i should find out the validity of the product key. what will happen if the product key is invalid? will i be allowed to download otha MS software or not? how wil it affect my system??