you can but you will have to delete the primary partition and create a new xp 1, do this from the xp boot disk. after that just quick format if your sure your disk is stable and run the xp installer.
make sure you have all the relevant xp drivers for your motherboard, gfx, audio and lan as xp doesnt auto configure like win7 does. b4 you start the install. on a dongle or disk.
if you cant find the xp drivers on your manufacturers site it means they dont support xp for that model so it will be pointless downgrading.