Mary added a second hard drive and it won't spin up. Leo says it's likely that her power supply isn't rated to drive two spinning hard drives. She could replace the power supply first, or try an SSD, as they take up less power. If those two don't solve the issue, then she's looking at a motherboard issue, but that's unlikely.
Jane's laptop consistently freezes up when she's doing things. Can that be caused by the battery? Leo says one way to test it is to remove the battery and plug it in to the AC adapter. If that resolves the issue, then it probably is the battery. But the battery is one of those that either works or doesn't work at all. It could be the power supply that is failing.
Roger's LCD monitor is "flashing" after about a minute and then settles down and works again. Leo says that it may be a bad cable, which would be an easy fix. But it's more likely that the monitor is aging and the transformer or power supply is starting to fail, requiring it to "warm up" before use. It's not really worth fixing. Monitors are so cheap now, Roger should probably just get a new one.
Moses has been getting a blue screen of death (BSOD). What is that? Leo says that a BSOD is a crash that has forced Windows to not work anymore. Moses has reinstalled Windows, but it still happens. Leo says that points to a hardware issue. A hardware flaw like a flakey power supply, video card or hard drive. It also could be a driver, but with a spontaneous reboot, it's probably the power supply. Fortunately, these are easy to replace. He can even do it himself. Moses should make sure he matches the existing power supply in wattage.
When Jeff tried to turn on his computer, it didn't do anything. No fans, no beeps, it was silent. He unplugged the computer, plugged it back in, and then it started up just fine. Since Jeff's computer is 6 years old, Leo says it may just be that the power switch is failing after years of use. It also could be a flakey power supply, or possibly something more serious such as a bad motherboard. Leo doesn't suspect the hard drive is at fault here.