|"You can modify boot.ini and add an entry to point to the Windows XP installation and it will come up as a dual boot configuration."|
This file would be erased when C: is formatted.
"I imagine Vista has something similar."
No, Vista's bootloader is completely different from XP's.
">. Also if I manage to find XP cd just run fixmbr right? <<
Actually, both fixmbr and fixboot. You run fixmbr first, then fixboot.
If you want to boot into XP, download Vista Boot Pro or EasyBCD.
Vista Boot Pro: http://www.vistabootpro.org/
EasyBCD is free, Vista Boot Pro used to be but isn't any longer. Like Stuart said, this would be done in 5 minutes with the XP CD.