Multiple Machines Not Load

I have been trying to get 1 of 4 machines to work. Scrappers they are. I got 2 machines up and able to see hard drives, but both will not load any WIndows OS. I tried 98, 98SE, XP Pro, and 2k Pro. Both machines will either crash during install, crash during the reg code (typing in the code and hitting ENTER), or crash at other points.
I replaced memory, hard drives, video cards, and everything I could in these machines but they want nothing more to do but crash.
I know it is not my CDs (no scratches or anything). The RAM is good, and one machine is using DIMMs and the other SIMMs. My NIC is good and my hard drvies are good. Most of this hardware is either NEW or from my machine I am currently on (because I got bigger drives).
Bad motherboards? Maybe. I have no idea.
Please give some input.