If you want to remove Win7 and install XP, sure. XP install will probably want to default to a floppy drive to get those SATA drivers and such though (they were not part of the original XP system).
If you want to keep 7 and use XP you'll need a new partition, install XP there, and use a dual-boot startup configuartion. Just make sure that the XP partition is big enough to be usable because of the additional software you'll need for XP. You can't start in XP and use a program installed while in 7 mode.
Others can be more specific if you elaborate on what you presently have and what you intend...
Before you mess around with installing XP on your computer, check the vendor support site if they have XP drivers for your model. If not, don't bother with it.
If you want to try anyway, make sure you make a backup DVD set of the original configuration.