Assuming you mean Windows XP (not EP) the answer is:
1. You can install Windows XP, but it will only work satisfactory if you install the XP version of all needed drivers.
2. You need to erase the disk with DBAN or KILLDISK first.
3. You only lose what you can't restore from your backup or can't install from the original setup program or disk.
So that's "yes, you can".
don't want to offend anybody but I hate vista it will run some of the old EP programs but not all of them, plus it takes it upon itself to foul up all the short cuts so that they don't work and the list of other things is too long to bore ya'll with... so can I take vista back to EP and not loose what I already have ??? thanks, dd