The MB and the CPU were pre-loved but reputed to be working.
When I first completed building this machine, I checked all the connections and switched on. The macjine switched itself off after about 15 seconds.
I was not sure how the previous owner might have set it up so I decided to wipe the CMOS by shorting the 2 pins on the MB - Clearing CMOS Jumper - to re-load the factory defaults>
I was then able to get the machine to BOOT, go thru POST and select BIOS Setup which I then saved.
From that point on the machine would usually BOOT, POST, get to setup screen and allow me to select the WIN7 Operating system disc in the DVD drive to start to load Win7 onto my SATA HDD.
The machine usually falls over after about 70 seconds after switching on.
The PSU appears to stay on after shutdown.