Short for Hardware Identification, a security measure used by Microsoft upon the activation of the Windows operating system. As part of the Product Activation system, a unique HWID number is generated when the operating system is first installed. The HWID identifies the hardware components that the system is utilizing, and this number is communicated to Microsoft. Every 10 days and at every reboot the operating system will generate another HWID number and compare it to the original to make sure that the operating system is still running on the same device. If the two HWID numbers differ too much then the operating system will shut down until Microsoft reactivates the product.
The theory behind HWID is to ensure that the operating system is not being used on any device other than the one for which it was purchased and registered. However, problems can arise when users start replacing or adding hardware components -- such as motherboards, sound cards, CD-R drives, Ethernet adapters -- because these will generate new HWIDs, and if there are too many differences from the original installation number the system will register that the operating system has been installed on a different device than the original and subsequently shut itself down. The only way to resolve the issue is to telephone Microsoft and explain the new hardware in order to obtain a new code.