Desktop computer is not running well

Desktop computer is not running well

The problem you are stating here seems like to be the problems with your memory devices. Though it is also very probable that your system is affected by a malware attack. Any virus that may be affecting your MBR (Master Boot Record) can cause the turning off of your monitor screen.

The issue of Non Volatile RAM missing" may be related to your hardware specifically NVRAM chip. The memory chip i.e. NVRAM might not be properly seated in its place in the motherboards.

There might be some loose points. The problem can be of overheating as well. If your system gets restarted over & over again then it needs to be cooled down. If at some point your system start at all & only a beep goes off then you need to clean your system CPU. Some moisture or dust may be clustered up in the CPU cabinet. 

If this all doesn't provide a solution to your problem then try installing a fresh copy of windows in your system. Take a backup of your important files of your system before proceeding. 

The problem of incompatibility of HDD with OS can only be solved with this solution that you find a compatible OS & then install it.

Hope things turn out well for you.