computer stops before ms dos can start

I belive the best way to learn how to do something is good old hands on training but my hands on is getting me no where. I have a computer (nothing fancy) that I use to learn on (ie; putting in hard drives, fdd, sound cards, just about anything you can put inside the case of my computer (which is a hp 486dx2 200 mhz) nothing fancy but easy to work on until now. whatever I have done I've done good. no matter what I change (hard drive, fdd, cables) and no matter how I make the computer boot up (by hard drive or by disk, as soon as the message "starting ms dos" it stops. regardless of what I try to do (f5,f8, hold the ctrl key down or the shift or change boot disks.) I can get to the set up screen to change system settings which are all set to default. but I can't get to the "c" prompt or bypass config.sys or autoexec.bat I have windows 95 and 98 on the various hard drives that I have tried, but again everything stops when "starting ms dos" starts. confused and hope someone can help.