I installed XP Pro (sp2) on a second logical partition (D
of my only physical drive. When it booted, it would pause during the DOS boot sequence and give a choice of which operating system to boot into. Did that so that I could copy my important stuff over to D:, then format C:.
Later, I found a better way: I put the My Documents folder on d: drive. Might try to move the 'Documents and Settings' tree over to D: next time I format.