I guess it has currently either Vista or Win7 installed.
go to the device manager and look up which video and network cards are installed. also you need to know the chipset.
next step would be to find all the XP drivers which will probably not be available from Dell.
the chipset drivers might be necessary to install XP, since XP usually does not support SATA controllers, and the NIC drivers to connect to the internet to update drivers.
the newer the machines the harder it becomes to find XP drivers