Hello antiherodave...
Licensing issues can be tricky sometimes.
Are the re-installation disks you have from brand name machines such as Dell, HP, Compaq, etc...?
If they are, these are referred to as OEM disks. The operating system and key from these disks are tied to the hardware on which it is originally installed and cannot legally be transferred to another machine. Another type of OEM disk is that which is purchased with qualifying hardware. These are usually plainly marked as OEM versions. They, too, are tied to the hardware on which they were originally installed. Essentially, OEM operating systems live and die with the machine they are first installed on.
If, on the other hand, your disk is a retail version that was purchased through Microsoft or another retail outlet, it's licensed to you and not to the hardware. Retail versions can be legally installed on any machine you own so long as it is installed on only one machine at any given time. These disks will be labeled with the Microsoft holographic label.
While a retail version may require a phone call to Microsoft for activation when you transfer it to another machine, it will activate. The registration number for the previous machine will be deactivated and the new one registered to you for the new hardware. You may do this as often as you wish.
As hfcg advised, an OEM version that is transferred from one machine to another will likely not activate with Microsoft...which will eventually lead to the operating system becoming nonfunctional. Even by phone, Microsoft will not activate an OEM version that's been transferred to a new machine. The only way around this is to use the original motherboard...then it will be seen as a hardware upgrade and not a new platform.
So...determining the kind of disk you have will also determine whether you can legally transfer the operating system to a different machine or not.
Does this help or just add to the confusion?
wannabe1