One option is, you'll need to set it up for dual boot.
When you install XP to a new partition, it will overwrite the MBR and prevent you from booting Windows 7. You'll then need to boot Windows 7 from CD/USB and regenerate the MBR - which will then allow dual-booting.
Another option is to install XP in a virtual machine (using Windows Virtual PC or Virtual PC 2007), so you can access XP and Windows 7 at the same time.
Both options have their respective pros/cons, so the better option is really dependent on your use case.