Does it fit this:
Error Message: The Product Key Used to Install Windows Is Invalid
or
"The Product Key used to install Windows is invalid" error message when you try to install a Windows update in Windows Server 2003
If it does, you most likely have a pirated Windows.
If it is pirated, you need to contact the seller and/or Microsoft.
For confirmation follow the instructions on the How to tell site:
How to Tell
If you suspect your version is not legitimate, contact Microsoft:
Reporting Microsoft Piracy
Service Packs may detect pirated copies of Windows.
Also you have 3 options to become legal:
1. Go back to the previous OS:
How to Uninstall Windows XP and Revert to a Previous Operating System
How to Manually Remove Windows XP and Restore Windows 95, Windows 98, or Windows Millennium
This may or may not work depending on the installation options used.
2. Perform a Clean Install with the OS of your choice destroying ALL data.
3. Purchase Windows XP Pro (Home will not work) and perform a Repair
Installation using the new valid key and CD:
How Do I Do a "Repair Installation"? (Windows XP)
Or
Purchase Windows Server 2003 the same version or above
How to Perform an In-Place Upgrade of Windows Server 2003