I assume it's a hardware error because nothing ever shows up in Event Viewer.
Sometimes when I turn the PC on it boots normally and I can use for usually only a short period of time before it locks up - no response from any keys or mouse, not even cntl/alt/del. Other times the monitor stays dark and I hear what sounds like a fan noise becoming increasingly louder and louder. In both cases I have to hold in the power button to reboot.
Sometimes after it boots correctly, simply letting it sit on the desktop will cause it to freeze. I have gotten it to run a chkdsk at boot - it found no errors, as well as a complete MalwareBytes scan - no errors there either.
I have unplugged and replugged the hard drive and its connections to the motherboard. The fans on the inside of the chassis are working
To make matters worse, I thought it might be a memory problem, so I removed a stick and it began to boot, but then I got the 'The amount of system memory has changed. Strike the F1 key to continue, F2 to run the setup utility'. This PC does not have serial ports for the mouse or keyboard - only USB; and the keyboard is non-functional at this stage of the boot process.
I even put the memory back, and am still getting the memory change message.
I really don't know how to get past that.