This isn't all that difficult.
Although I don't know the exact parts & wiring, you can pull this off with relays. The PC itself can be setup to power on as soon as it receives power. The setting for this is in the BIOS. Wire the power buttin for the PC to wherever you want it. With WinXP, a simple press of the power switch will initiate a proper shutdown.
For speed of startup/shutdown, you might want to consider hibernation instead of full startup/shutdown.