shutdown then restart

My system's never done this but I see it is quite common from a simple DEJA search.
Products reported to cause this issue are Quicken and Logitech keyboards.
Most answers resort to reinstalling the OS, however, chack with the manufacturer for the latest BIOS updates.
A little more system info would help too, as well as when did this start happening? New software, all the time, last BIOS update?

A couple of things to try.

1. Install Service Pack 3.

2. You may have an issue with ACPI (Advanced Configuration and Power Interface) compatibility. Right click on My Computer - Go to Properties - Go to Hardware tab - Go to Device Manager - Expand the Computer entry (click on the +). It should read "Advanced Configuration and Power Interface (ACPI) PC". If it reads "Standard PC" then the computer BIOS was not detected as being ACPI capable. If that's the case, you'll need to flash (update) your system BIOS.

I had one do this. Shut down, then some random time later - could be 10 seconds, could be hours, teh computer turns itself back on. As I recall it started happening after a BIOS update I installed to fix a problem with printing. Gigabyte motherboard, Athlon processor, XP home. I think I finally fixed it by making sure all updates were installed and also making sure the power management settings were correct. Can't remember the exact fix because I tried a couple of times.

Try this - get a DOS/Win9x boot disk. Boot from the disk to DOS, then shut the computer off. If it stays off, then it is a Windows problem. Check power management, 'wake on' settings, etc. XP is setting something in the CMOS config that makes the computer turn itself on.

Not To Overlook The Obvious,
In The Shutdown Screen Are You Checking Shut or Possibly Re-start.

bios is up to date.
acpi is enabled in bios and windows 2kpro.
shut down screen is checked.
tried boot disk then manual shutdown still reboots.