If you replaced your Compaq's motherboard with a non-Compaq motherboard, the OEM product key affixed to the Compaq computer can no longer be used.
An upgrade of the motherboard is considered to result in a "new personal computer" to which Microsoft OEM operating system software cannot be transferred from another computer. If the motherboard is upgraded or replaced, for reasons other than a defect, then a new computer has been created and the license of new operating system software is required.
The replacement motherboard must be the same make/model or the same manufacturer’s replacement/equivalent, as defined by that manufacturer’s warranty.