You may have a trial version of office 2007 already on the computer. Go to control panel > uninstall Applications > and remove any microsoft office programs > then try the install again. Also you may or may not run into this problem but he tells you how to fix it if you do.

http://uksbsguy.com/blogs/doverton/archive/2007/03/12/loading-office-2003-into-vista-and-fixing-the-problem-with-the-eula-always-coming-up.aspx

Good Luck HTH