Yes you can, but installing XP after 7 will cause the bootloader to mess up for Windows 7. You can fix this by using Easy BCD to rewrite the bootloader and getting everything back into place and working.http://neosmart.net/dl.php?id=1
and download the disc image.
Now, if you install XP first and then 7 after, the newer OS will actually update the bootloader properly so it all works. That, however will involve you reinstalling Windows 7.
A: Install XP to 2nd your drive and use Easy BCD to fix the bootloader
B: Wipe everything off your system and install both operating systems, starting with Windows XP