When assembling the system. Connect only on HDD, install Operating system. Verify good installation (Note Does NOT matter which is first XP or Win 7). Power down, disconnect that HDD and connect 2nd HDD. Now install 2nd operating system. When done. Power down and reconnect the first HDD.
When you power on and system is in post Hit the Key that brings up the Boot menu and select which HDD to boot to (on My gigabyte MB it's F12).
If, it is a"store bought" system with say Win 7 installed, Just follow the method above - disconnect HDD w/win7, install 2nd HDD and install XP.
Above method does not require a software Boot managet - A great plus. Which ever drive you boot to will be your "C" drive, and Unless you partion the HDD the other drive will be "D" and fully accessible.
Had my system with 3 sets of HDDs, vista, XP, and Win 7 RC - worked great.