<<The hard drive is from a laptop put into a desktop, and keeps claiming that our "copy of Windows is not genuine">>
The hard drive from any laptop...since all laptops are OEM...contains the settings/drivers, etc. for a specific system.
Moving that hard drive to a different system...as a boot drive...is a violation of the Microsoft EULA, since the license data relates to a system which is not the one you installed the hard drive into.
The only situations in which this would not apply would be if you are using the laptop hard drive merely as atorage, secondary hard drive...or (possibly) if you move the hard drive into a system which has the exact same hardware and is made by the same manufacturer.
Changing a Motherboard or Moving Hard Drive - http://www.michaelstevenstech.com/moving_xp.html