You can use a repartitoning software to shrink the Win8 partion and create a new partition out of the free space.
Then install XP onto the new partition.
Transfer your files from the Win8 partition to the XP partition from Win8.
Then boot up into XP and use the repartitioner again to delete the Win8 partition and then expand the XP partition to the full capacity of the drive.
And then you will have converted system back to XP.
Simple Right ?!!!!!!
FWIW
jer