contains a fresh installation of XP Pro on a computer that previously had no shut down "problem" I would suggest that you take a loop in the Control Panel for the Power Settings. This assumes that you did not make any BIOS alterations.
What you have described isn't really a "Shut Down" problem because it has shut down - it is a Power Down issue.
"When Windows XP won?t powerdown automatically, the APM/NT Legacy Power Node may not be enabled. To enable this, right-click on the My Computer icon, click Properties | Hardware | Device Manager | View. Check the box labeled ?Show Hidden Devices.? If it?s available on your computer, there will be a red X on the APM/NT Legacy Node. Try enabling it and see if this resolves the powerdown problem (Tip from Terri Stratton). Or, to check the other side of the APM/ACPI coin, open the Power Options applet in Control Panel. If there is an APM tab, make sure the ?Enable Advanced Power Management Support? box is checked."
several other recommendations on that page.