In Win2K, I always do the following when swapping out the motherboard:
1) Ghost my C: drive onto an old hard driev I have lying around, just in case I need to roll back.
2) Install the new motherboard and see if things work. In many cases, Win2K detects new devices and things work OK, and no additional work is necessary.
3) If windows doesn't work, then I do a "Repair-Install" which is a Win2K installation over the old one. This type of installation preserves all of your settings and all old programs work normally. You just need to re-install all servie packs and windows updates to get the system fully back to normal. Overall, its pretty painless.
I don't know if Win XP has a similar feature of allowing you to do a repair installation. When going through the installation in Win2K, you first say "no" to the to the option of performing a repair. Then when it detects that you already have win2K, it will give you the option of installing a fresh copy, or repairing the existing version. Choose to repair, and you are good to go.