What version of Office do you have? What does the error message
say exactly and completely?
If you have Office 2003, and if your computer has a free trial of
Office 2003 installed on it, then you need to uninstall the free
trial software before you install your purchased software. See
http://support.microsoft.com/?kbid=836178 "You receive an
'invalid product key' error message when you start an Office 2003
program after you install Office Professional Edition 2003" for
more information.