Motherboard

wrong section, it might be the power supply you installed being a dud try reinstalling the old back in and if it works then the new one is bad.

easy peasy

From what you described that is all the help I can give.

This topic has been moved from the section Windows XP to section Motherboards & Memory by Mrface

give some more hardware info like PSU, MOBO, or Computer Model if it is a brand name