I now have XP 32 bit and Windows 7 64 bit on One HD.
The main thing about doing this is unless you have XP/vista 64 bit you can not put a Windows 64 bit on the HD.
I first put a partition on the HD. Then took out the HD and put it in a 64 bit PC (MY wife's) then installed windows 7 32 bit first you must do that first . Then the Windows 7 64 bit installer and load to the Windows 7 32 bit and unguarded it.
Put my HD back in my PC and booted up. The PC asked if I want to run the older version windows (being XP 32 bit) Or Windows 7.
After windows 7 loaded it was 64 bit.
flyer01