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.
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.
So...
A: Install XP to 2nd your drive and use Easy BCD to fix the bootloader
or
B: Wipe everything off your system and install both operating systems, starting with Windows XP