The probable problem is that the motherboards have chipsets from different manufacturers. When you insert the disk in the second computer, the IDE drivers residing on the disk do not match the IDE controller on the motherboard. Therefore, windows has no way of actually "reading" the hard drive. When you install Windows, it detects the chipset residing on the MB and installs the correct drivers.
You therefore have two choices:
1. Fdisk/reformat/reinstall - probably not the way you wantto go or you wouldn't be asking this question.
2. Place the drive back in the original computer and follow the directions in Microsoft Knowledge Base Article 314082. Basically, what you will do is make some load some files generic drivers for MOST coomon chipsets) from the Win2K installation CDs and make some registry changes that will cause Windows to detect the chipset and use the appropriate drivers upon boot.
(NOTE: If the reason you changed computers is because the first one was kaput, you may have no choice but to use choice 1.)
Hope this helps.