1.The HAL, Hardware Abstraction Layer, varies from one hardware set to another.
Typically, you have two. one for single processor and one for multiple processors.
Obviously if you are moving to another architecture the HAL will be different. (with the exception of moving from x86 to x64).
2. Specific drivers for drive controller, video graphics, network and audio differ.
The result being that when you first boot that system, none of those drivers load. There seems to be an assumption that if you install a driver, it will automatically load. It only loads if the DeviceID for which it is associated in the CurrentControlSet is found to exist. Otherwise, it isn't loaded.
3. Microsoft XP is licensed and activated for just one hardware set, according to the EULA. But if you buy an identical motherboard it may work.
OEM Windows is licensed for one hardware set, and typically the software activation is handled by SLIC or other BIOS integrated capabilities, making migrating the install unfeasible.
Retail, as I understand it, is licensed and activated for one
machine. The system needs to detect a few changes before it will need to be reactivated again. Sometimes a motherboard with a lot of integrated components can set it off.
We still don't know what this "system disc" is. a Windows XP retail? OEM? a factory restore disk? etc.