Don't Tell Him Pike,
With WinXP, should the install ever fail, you can always do Repair with the WinXP CD. If that does not fix the problem, you can always do an install of WinXP on top of itself. That should solve your problems and as long as you tell WinXP to keep the current File System intact, you will not lose any data short of what you had in the Windows Directory.
So you can do an install on the WinXP Hard Drive and not have to worry about having your data on the same Drive.
Take Care,
Richard