I've been experiencing this a bit with a few of my systems. One is a T-bird 800 on an Asus A7V and the other is a Coppermine 750E on a MSI BX Master. Both are running Win98SE. The problem is that if either system goes into standby (if the monitor blanks out from inactivity), either I can't get the display to come out of standby or when I shutdown, it reboots.
The way I've worked around it is to disable all power management. Right-click on your destop, select screen saver, click on settings, set power schemes to always on, system standby to never, turn off monitor to never, and turn off hard disk to never. Then set a screen saver. Doing this, I never have the monitor blanking out on the Intel system or the reboot on shutdown with the AMD system. Of course, I'd love to find a better fix than what I'm doing now.
If you're having problems shutting down immediately after booting up (i.e. not experiencing any type of standby or sleep mode), you have a worse case than I had.
As for shutting down via power switch, when I was having the reboot on shutdown problem on my T-bird system, I'd let the system reboot and press "control-F5" once all the CMOS stuff was checked. Then I hold the power button for 4 seconds to shut down the computer manually. You could also press F8 and select the command prompt option and then shut down manually from there. As long as it doesn't get to loading Windows, turning off the system manually with the power switch isn't a bad thing.