Symptoms of a Bad Power Supply?

Hello,

I was away for a couple of days and my wife was home with a couple of browsers open and had just checked her email. She left the room for a a bit and came back to a black screen. Usually on my monitor power light, it's amber if in sleep mode or green when it's on and okay. It was green but otherwise, the computer wasn't responding as if it were in sleep mode and wouldn't come out.

I got home and everything appeared normal other than the black screen (keyboard lights on, computer power light on, monitor green power light on, etc). Since there was no way to reboot the computer (ctrl-alt-del or start-restart), I ended up turning the box off with the power button. Upon reboot, it showed the usual Microsoft Windows XP with the blue bar moving left to right but it was obvious after half an hour or so, it wasn't going to load the registry and boot up. Pretty much ditto when resetting and trying any of the safe-mode settings. It was frozen. On one occasion in safe-mode, I was able to get it to load the user screen and get my administrator password entered but after I clicked enter, it froze again.

I'd considered some sort of spyware, worm, virus, etc but am inclined to think it may be power supply related. I haven't ruled out the motherboard or hard drive either but the HDD is fairly new (Seagate), no hardware changes have taken place for about a month (when I replaced a CD drive), and the machine was scanned a week or so ago with several virus/malware/spyware scanners.

When booting, it runs as normal up to to the boot screen where MS Windows XP Pro is advertised w/lively blue bar running left to right but it never does get past that. I booted it to safe mode and left it on all night only to check it this morning and note that it had rebooted over night on it's own.

It's a Dell Dimension 8400 w/newer HDD and a power supply that was replaced about 4 years ago. The computer is about 6 years old. I'm thinking about replacing the power supply with one I have just to see if the symptoms change.

Any ideas and thoughts are greatly appreciated. I'm hopeful to have my computer back soon.

TIA