Now before we proceed I need to know some data
Is it an OEM?
OEM Software-Original Equipment Manufacturer something like that came with your branded computer/laptop like a Dell or HP or so.
If it came with buit in Vista through such companies make sure there are all XP compatible drivers available for that computer before you recovery or else you might end up without audio or proper video drivers after in going to XP.
In case it came with XP but was upgraded to VISTA then no issues insert an XP cd or recovery cd and wipe out evry thing on the disk (try repartioning by taking out the partion and then allocating a new one..simpler than it sounds just watch the blue recovery screen on XP cd and you will know)
So may word
If it came with VISTA: check and confirm that drivers are available for that motherboard in XP too and you have them with you.IF you got all reinstall using XP cd.
If it came with XP and was upgraded go ahead and Reformat and reinstall.
To do that Installation after confirming the drivers
Insert the XP recovery or the RTM disk
once this is inserted
restart the computer and wait till it prompts to boot from cd
(if you did not get that prompt or message you need to assign boot order:check the BIOS/motherboard manual)
Once you have selected to boot from that cd
it will check and l;load all sorts of things for a few minutes..wait
Now it will show you th EULA (End User license Agreement)
Press F8
again some delay and the drives are listed with current insatalled windows
select the one that has windows using the arrow keys and then read the options given at bottom and then proceed to reinstall or delete the partition
once done its very easy proceed in the same line