Installing XP over Vista !!!!

You need empty partition . To make one use computer management to resize existing partition and make space for a new one.
To resize partition :-
Go to disk management application . Righ Click on partition .
Click on shrink volume. Choose SPace.
Change drive letter or path to avoid confusion.
Right click on empty space created.
Click on new Simple volume.
Create partition with right click drive letter.
now --- > Boot using XP installation CD and install.
--- > You wont be able to boot into windows vista any longer.Boot into XP and install software called VistaBootPRO from www.vistabootpro.org which allows to make entry and changes to boot record.