Were you thinking of daisy-chain powering the LCD monitor through the computer PSU? (power supply unit)
This used to be the way of things in combination with an IEC male/female power cable [
like this] but has been phased out for many years. It's possible that the PSU on your aging P3 might permit this kind of configuration. This isn't current practice and places unnecessary load on the computer PSU.
As others have said:
- You will need the appropriate drivers for the monitor (XP may well detect it, but it's possible that it will simply show as "generic"). You will almost certainly be able to download these.
- As long as the LCD has a VGA [analogue] connector rather than, or in addition to a DVI [digital] connector, it should work fine.