I assume you want to do this because you're having some sort of problem. If that's the case, there is a built in method of doing an overlay installation - it's a repair install.
Just boot to the XP cd and it will immediately recognize the current installation and offer you the choice of either continuing a full installation or doing a repair. Choose the full installation. Two screens later you will again be offered the option to repair. This time, select that option (press "R"). XP will then perform a full installation on top of itself and your registry entries should not be modified.
Remember, choose the second repair option, not the first (the first will drop you at the repair console).