Let me ask this. Are you cloning XP from Win7 and then trying to boot XP and it isnt working? That will happen. Simply put your copying the wrong boot files. You are copying the boot files for XP as Win7 sees them, not as XP sees them. So of course you wont be able to boot to XP.
If you want to clone XP and have it boot, you have to first be within XP and then clone it so it takes the right boot files with it.