Re: New Motherboard with old hard drive
To perform the "repair" install that linderman recommended - you'll need the a CD with the same version of XP that was originally installed. There are many different flavors of XP (Home, Pro, OEM, SP2, etc..) so having the same version is essential.
Have you tried bringing it up in SAFE mode on the new motherboard (press F8 before windows boots and pick SAFE MODE)? I've actually been able to bring systems up in SAFE mode, delete all of the incompatible hardware in Device Manager, restart and let Windows auto-detect the new hardware. This is NOT foolproof - so I only offer it as one possible approach.
If you can't get ahold of that XP CD, you may end up installing XP from scratch. This will overwrite all of your data, settings, installed programs, etc. so make sure you have backups of your data files, as well as install CDs or other means of re-installing your applications. Also - make sure you download any hardware drivers specific to your new motherboard - i.e. graphics drivers, network card drivers, sound card drivers, etc. Save all of this stuff on a flash drive or burn it to a CD or DVD.
A fresh install might be more time consuming and tedious considering all the drivers, apps and data you'll need to re-install - but this approach will give you a cleaner running copy of Windows that's specific to your new hardware.