well, you can try "sysprep" on it, that works for me when i have updated hardware and don't want to do a full reinstall. BUT, if it doesn't work, you will then *have* to do a repair install. (if you're reluctant to take the chance, a repair install is the safer way to go; it accomplishes the same thing, it just takes longer.)
as the previous reply said, the *best* route is a full reinstall, but if you have a complicated setup with lots and lots of applications, that's a real PITA.
http://support.microsoft.com/kb/302577/en-us
is the link to the KB article describing sysprep. the utility is located in the SUPPORT folder of a Windows 2K/XP install disk. there's a folder in there called TOOLS, and inside there are some files. the one you want is called DEPLOY.CAB.
if you extract it, you will find a number of files, including sysprep.exe and setupcl.exe. create a new folder in the root level of your C: drive called "sysprep" and put sysprep.exe and setupcl.exe in there. then run the program in the DEPLOY folder called "setupmgr.exe". this will create the .inf file sysprep needs to get it's information from. it's pretty self-evident what info setupmgr wants: choose "Create new", "sysprep setup", pick which windows version, and choose "Yes, fully automate the installation." (though you can do a non-automated one and answer all the questions during the setup process, like you do with an install or repair.)
from here, answer the questions as needed. most of the stuff you won't need to add anything to, but the name, computer name, administrator password, and product key fields you will want to answer. (magicaljellybean.com has a keyfinder utility that makes it easy to get your installed windows key if you have no other record of it.)
when setupmgr finishes, it creates the sysprep.inf file. place a copy of this in the C:\sysprep folder. open a command prompt and change the directory to "C:\sysprep". type the following command when you are ready to shut your computer down before the upgrade: "sysprep.exe -reseal -mini -pnp".
this command forces sysprep to use the NT mini-setup (the question section of a windows install) and to re-initialize the Hardware Abstraction Layer (HAL) (i.e. the PnP info for your hardware.)
sysprep will "work..." for a bit, then shutdown.
when you reboot, you will eventually see a screen that says "please wait while setup is working" or something to that effect. eventually it will finish and continue to boot into your windows environment. at this point, you should be back to where you were before, with a fresh HAL and a new activation to go through!
if you're lucky, you will activate with no problems. you will need to run Windows Update again if that's your thing and probably have to restore a bunch of interface options, as these are reset during the sysprep run. also, install the motherboard drivers from the disc you received with it.
it's a *really* good idea to backup your current installation somewhere before you start this, as things can go awry and leave you with no choice but a repair install, or worse, a full re-install. i recommend moving all data files to another drive or partition (music, videos, documents, installers, etc.) and/or running the Files and Settings Transfer Wizard on your system and moving the USMT.UNC folder onto another drive as well. this will allow a quick restoration of most of your settings (and files if you choose that route.)
i use this procedure pretty frequently at my work to upgrade systems while leaving the windows installations intact. it saves a great deal of time on the 90% of systems that have no problems. i had one big hassle with a system the other day because it had been partitioned with "System Commander" and had a boot manager installed in the boot sector. this prevented windows from booting correctly and caused a great deal of time wastage.
good luck, if you choose to accept this mission.