How do you determine what is broken?

Ok, so the last two PC's I built were back in 2002 or so. They were/are P4 1.6GHz, on an Asus Th7II with 500gb of Rambus.

Ran Great for about 3yrs. Then both quit at the same time.

Regretfully I had unknowingly bought a copy of XP Professional off eBay that was that infamous pirated corporate one at the time I built my PCs. It came as a University of Illinois student version. I did try to return to the seller when it came but he was taken off eBay right after the sale and nothing ever came of my complaint to eBay.

Anyway, I did not know when I started using it that it was the pirated version. But eventually found out from references to the Product ID that seemed to say F____ Bill Gates in the Product Key.

Could it have been the XP updates or a virus that broke my PCs? I have had these PCs sitting unused for about 5yrs now. I thought maybe it was the HD so I bought a new one and tried to install a legit version of XP and it did not work. I get the Bios screen but it won't install XP.

I should mention, even though the TH7II was known as a good overclocker, I never felt a need to OC for what I was doing, so I don't think I was overheating things at all. I had a ton of fans running in both. So I think that could be ruled out. I was doing a lot of web design and photoshop at the time though.

I am curious, how do people figure out what is wrong with a PC when they just quit?

I did an FDISK with a floppy I had first. And yes I did that too.

I "was" pretty knowledgeable about how to set things up. I had built few before that and knew my way well around the Windows OS and install procedure having started with Dos, then Windows 3, 95, 98, 2000 etc.