I have XP & windows 7 .You need to format drive then partition it then install XP FIRST then install Windows 7 * then your will have the option of booting to "earlier version of windows (xp) or Windows 7.
Another option (probably more suited to you is to d/l EasyBCD http://neosmart.net/dl.php?id=1
EasyBCD is NeoSmart Technologies' multiple award-winning answer to tweaking the new Windows Vista bootloader. With EasyBCD, almost anything is possible. Setting up and configuring Windows boot entries is simple, and there is no easier way to quickly boot right into Linux, Mac OS X, or BSD straight from the Windows Vista bootloader - on the fly, no expert knowledge needed!
EasyBCD is geared for users of all kinds. Whether you just want to add an entry to your old XP partition or want to create a duplicate for testing purposes; if you're interested in debugging the Windows Kernel or septuple-booting your seven test operating systems, EasyBCD is the key.