PC won't turn on without unplugging/ re-plugging power cable

From: XP user (anonymous_at_discussions.microsoft.com)
Date: 05/03/04 

Date: Mon, 3 May 2004 15:36:03 -0700

I just installed a Radeon 7000 card & new monitor, and now my machine does not power-on after shutting down.
To turn it on, I have to unplug the power cable, wait a few seconds, plug in the mains & then press the power-on button. (My PC doesn't have a power switch on the back near the power supply)

On the BIOS power management set-up, I have ACPI enabled (the computer hangs if I disable ACPI here)
Any help would be greatly appreciated!!