Try this for a first effort... I have an Acer desktop that I am attempting to diagnose. It will just switch off randomly, IF I can get it to start at all.
It has a SATA HDD, SATA DVD drive, 2 GB RAM (2x1GB DDR2) and some sort of Pentium CPU. At times, it will boot, tell me that Windows (Vista - yuck) failed to start correctly, and needs to be repaired. I haven't yet managed to stop it and boot to a DVD at this time... At times, when it DOES boot it will sit just waiting for me to log into the repair console - WITHOUT shutting down. Most other times, I will switch it on, the fans will start up, then keyboard will light up, and the fans will then go off, alas, the machine has powered down. I can't get it to the point to run UBCD over it to test the hardware.
I have replaced the PSU with a new one, same problem.
I have unplugged the HDD, same problem.
I have replaced the RAM, same problem.
I have unplugged the DVD drive, same problem.
Am I right in assuming that this is a CPU/MB issue? Anyone else seen these symptoms?
Any insights greatly appreciated. Frustrating as I can usually diagnose these, but I can't even run a CD/DVD!!!