Hi Ray,
Which operating system is installed on your machine at present?
a) For installing Windows XP or Windows 7, you may follow this link: Install, reinstall, or uninstall Windows
b) Thereafter, try to activate Windows XP or Windows 7.
For Windows XP, refer: How to activate Windows XP
For Windows 7, refer: Activate Windows 7 on this computer
If it doesn’t help, you may contact the Microsoft Product Activation Center for assistance.
http://support.microsoft.com/kb/950929
Hope the information helps.
Please post back and let us know.