if you just want a newer faster drive in the old system just clone the drive to a newer bigger HD then unplug the old drive...
it won't work if you try putting it in a new system though because of the change in hardware like you said earlier
Windows really doesn't like having the motherboard changed underneath it and will generally refuse to boot at all and if you do get it to boot...with enough changes you will have fits trying to activate it...see the list below on what it checks...
the 10 things XP hardware does Product Activation list and check...xp reactivation
Which items of a computer's hardware does Product Activation list and check?
WPA checks ten categories of hardware:
1. - The display adapter - the video card.
2. - SCSI adapter - if present it allows the use of SCSI hard drives and other SCSI devices.
3. - The IDE adapter (the motherboard's IDE hard drive controller)
4. - Network adapter (NIC) and its MAC Address
5. - RAM - amount and range (i.e., 128 to 256MB, 256 to 512MB, etc.)
6. - The processor's type - the make and model of processor(s)
7. - The processor's serial number [not all processors have serial numbers, and some that do allow it to be disabled so that it can't be read]
8. - The hard drive device
9. - The hard drive's Volume Serial Number (VSN)
10. - The make and model(s) of CD/DVD drive(s) installed.
The make and version of the BIOS doesn't count at all. The product-activation code is only locked on to the BIOS with the OEM version of Windows XP that the major computer manufacturers pre-install on their computers.
The activation process calculates and records a number based on the first device of each type that was found during setup. The number is stored on the computer's hard drive. During the actual online activation the number and the Product ID number, which Windows creates from the 25-character unique Product Key that every non-corporate copy of Windows XP has, are returned to the activation centre. If you bought a brand-name PC, the Product Key is usually attached somewhere on the computer's case. If you have bought a retail or OEM version of XP, the Product Key is supplied with the CD. If Service Pack 1 or Service Pack 2 has been installed, the entire Product Key is also transmitted in order to check it against a list of known pirated Product Key numbers.
Microsoft keeps a record of the information it collects on every licensed copy of XP so that no one can install it on another computer and activate it. It would be a prohibitively costly business to create a computer that has all of the same hardware, MAC Address, the Volume Serial Number, processor serial number, etc., as another computer in order to be able use one copy of XP on more than one computer.
If you format the hard drive (usually the C: drive) and reinstall of Windows, reactivation is usually required because the new installation contains no activation information. But if you do reinstall and reactivate XP, if the hardware is unchanged or substantially the same, the activation will go ahead as easily as if that copy of Windows has never been activated before.
How does the process work out what substantially the same means if you have made changes to the system? Well, believe it or not, it asks for a certain number of yes answers to questions it poses on each of the ten categories of hardware that are taken into account. For example, it checks the Volume Serial Number of the hard drive with the current one. If it has changed that is a No answer. Seven Yes votes are required in order to avoid reactivation if the system has changed. A network card (NIC) that was present, and is still present, counts for three Yes votes. Minor adapter cards, such as a sound card, don't count at all. You can install as many of them as you like without having to reactivate Windows XP.
In other words, the same motherboard, RAM, processor, and NIC, provide enough Yes votes to enable you to make any other changes without having to reactivate. If you change the device in any particular category, you have lost its Yes vote, but that category of device can't lose a Yes vote more than once, so it can be changed as much as you like without a penalty thereafter. For instance, after initially changing the video card, you could install a new video display card every month for as long as you like without being penalised in the vote count. However, if you boot the system in which a device that has been disabled, the device won't be found during the check that takes place every time the computer is started. For instance, if you disable the network connection that uses the network interface card (NIC) and then reboot the system, the missing three votes that it provides can make reactivation necessary if the count of Yes votes goes under the seven required.