I have done this 4 or 5 times. Only one time did I have to format & reinstall Windows. In that case I suspect the new motherboard, as it was flaky even after the restore.
The one big problem can be if your Bride's computer is a manufactored machine, the restore disk it came with most likely won't work any more. If this is the case, in the future you will need to use a straight Windows CD. It also may not work for the board's installation, so you may want to get your hands on the proper Windows CD for this.
I always go to device manager and remove everything first, then shut down. Then I replace the motherboard minus any modems, sound cards, Lan cards, ect... This helps keep things clean during the boot up as the new board is being detected.
When you first boot up you will have to set the bios of the new board, date, time, ect..., if the new board has onboard Lan, Modem, or Sound, be sure to disable them.
Then reboot and the installation should begin. You will notice it asks for drivers from your CD ROM, but you will not have a detected CD ROM the first boot. Point Windows to the system files for these. Once your IDE is detected, you can continue to reboot and use both the motherboard CD, and your Windows CD to complete the installation.
Once the board is loaded, install the video drivers, then install the rest of the cards one at a time.
The worst that can happen is you will have to do a restore.