To avoid trouble and having to do a lot of work arounds to get things running properly if it is a fairly new install of win 7 and you have backups of your programs - Best thing to do would be start over and install the older operating system first !!
The reason for this is when you install XP it is not aware of how WIN 7 sets things up and will mess up the win 7 boot loader and install its older version and make XP the default OS -- and you have to go through many steps to get the WIN 7 Boot loader working again so you can select which OS to boot into on start up. While iff you install XP first and then install windows 7 afterwards it recognizes the win XP install and will properly update itself to include the boot selection options when it installs (as long as you tell it to do so when installing) -- So in the long run it is easier and less likelly to encounter problems later if you just start over and install XP first and then install windows 7 afterwards.
IF you do not want to do it that way (if it is an older install of win 7 and starting ffrom scratch will present difficulties) then HERE is a tutorial that details the method for installing win XP to an existing WIN 7 system and getting the Win 7 Boot Loader working again using a third party program that might be useful !