Computer shuts down.

Have upgraded from Windows 98 SE to 2000 Pro. Half the time or more when I first power on my computer or do a reboot, if I choose to go on the 2000 system, the computer completes the Welcome Screen, then powers down the computer. I try one or 2 more times and it eventually launches normally. Tried another power supply and does the same thing. When I go on to 98, it boots normally.