The easy way is to install XP first as then the dual boot is set up by Windows 7!! I have a dual boot with separate partitions for personal files ( documents,letters, and the like), Pictures , Music etc. These can be accessed from whichever OS I boot to. W7 only has the programmes it uses installed on that partition. It doesn't matter which you install first. Once you have installed XP, run EasyBCD and you can set the boot window. I use an 'old' free version of EasyBCD which works perfectly.
This guide may help click here