Most of the tiime the need for reinstalling Windows is essential since a board swap is a major component. You want XP to have a fresh and full detection of the new board, a new registy as well as old drivers removed, and Windows will load faster with the clutter in the old one now gone! Formatting may not be needed especially on a new system where the current partition is intact.
The installer will simply delete the current installation before proceeding to redo everything from scratch. Whatever you have in the MyDocuments and other sub folders in Program Files and Windows directories like wavs, jpgs, bmps, gif files, photos, and other items even mp3s you want to copy or move to temporary folders outside of those for safe keeping.