Acts Like Fauty Power Supply But Is It?

I have a packard bell multimedia 601 Pentium 233Mhz. I have a 204 watt power
supply. When I first got this machine there was a problem with the power
supply so I got the one I have in the unit now from a friend with a similar
model. It worked find. The power switch was not used, the machine was turn
off at the outlet switch. This machine was used by my parents. I replaced it
with a P2. I wanted to use the Pentium to run some older programs. I
installed a hhd, cdrom, floppy drive, from an old 486 I had. When I powered
it up it worked fine as far as power goes. In fact I was surprise I didn't
have any problems after hooking every thing up like I often do when working
on these things. The machine was configuring the new hardware, but the mouse
would not work with that machine, and I wanted to format the drive so, I
shut it down with the power switch on the machine to put in a boot disk to
format the hhd. When I when to turn it back on with the power switch ......
nothing. I could see the power supply was drawing current because the light
in the room would dim a bit. I would also see the fan move slightly, and a
light blink on the cdrom. Other than that nothing. I had no indication what
so ever of a supply problem. It worked fine for the months my folks used it.
My friend that took the original power supply for this machine told me he is
using it and it works fine! So with the information I have provided is it
possible for a problem to look like trouble with the power supply but be
something other than that? Help please?