Cold boot problems

I have Windows XP Home edition sp2 installed along with all the security
updates provided by Windows Update.

My installation is a clean install (formated drive) onto a system that
previously ran ME.

I am having a strange cold boot issue. My computer will not boot from cold
unless it has been disconnected from the mains supply for a few seconds
(restarting is fine). If this hasn't happen the machine won't post and the
gfx card does not initialise (cutting the power at this point and
re-supplying it causes everything to boot normally).

This isn't a psu issue (I have a 550W unit), the boot from the XP CD and
install process went perfectly and restarting doesn't cause a problem, the
system behaved normally under ME. It only happens when the system is
shutdown. The shutdown process appears to work correctly, I hear the drive
heads parking and power is then cut.

I have no warnings in device manager and all my devices has driver correctly
installed, I'm running the latest motherboard bios and chipset drivers (SIS).
I have ACPI enabled in the bios.

I'm guessing at some sort of ACPI issue where the system doesn't shutdown
correctly but I have no idea how to go about fixing it. Any help would be
appreciated.

With Thanks
Paul