Dave's wife got a "blue screen," and he thought the monitor was the culprit. Leo says the simple and cheapest solution is to swap out the monitor cable first. If the issue persists, he should try updating the drivers. He can create a bootable thumb drive that will allow him to get into the computer without running the hard drive. He can also do it from a CD. His recovery disc will do this as well. Then if it's a driver, booting to a separate OS would tell him a lot. If it's still blue, then he'll know it's a hardware issue.
If Dave can get into the BIOS settings, that's another way to tell if it's a hardware issue. If he can't, maybe the video card is failing or the motherboard is dead. It could even be a corrupted BIOS.