How do I assign different keys to 'Wake up" and 'Shut Down" in Windows XP?

Depending upon what keys they are, you can install KeyTweak and use that to modify the key. If it’s not a key that shows up on that key map, then you can simply assign a couple of F keys and abandon the use of the original key.

What OS version.
It is usually found in Start | Settings | Control Panel | Power Options | Advanced

You can set the Power Buttons options there on what to do when certain things occur:
When I close the lid
When I push the power button
When I activate sleep

There also may already be a Function key assigned to this that you can change in the BIOS.

To keep it simple, use the Windows OS control unless you need to get more granular.