clipped from: www.pcmag.com 04.04.08
Fed up with Vista? Here are step-by-step instructions for downgrading back to the warm familiarity of Windows XP.
Problem is, downgrading a Vista PC to Windows XP can be a chore, and doubly so if you want to do it without reformatting your hard disk. The main roadblock is Windows Setup, which won't let you install an older version on top of a newer one; you are left with the tedious task of fooling Setup into thinking that there's no Windows at all on your PC.—Next: Regress Without Reformatting >
Before you proceed, make sure you have several gigabytes of free space on your boot drive (usually C:). Take a moment to back up your entire hard drive; if you have Vista Business or Ultimate, you can do so with the Complete PC Backup and Restore tool, making it easy to get back into Windows should this process hit a snag.