Maybe I wasn't clear enough.
The only way to find out what is wrong is to try, your CPU, HD, and memory in another computer, or try known good parts in your computer.If all the parts check out it's the motherboard.
This is how any computer shop would do it, there is no quick diagnosis in a Toshiba service manual.
You are not wasting our time, that's what we are here for, but if you don't want to or are unable to follow our advice, that is no reason to question what we are here for.