Desktop PC not booting

[hypernova]

...Eh, so I'm having that problem.

It will boot the initial bootup screen, and I can modify the mobo bios settings and such, but won't get much furthur.  Here's what happens.  When loading from the HD, I can't get to the PCI device listing screen.  I did manage to get it to show up the OS options screen (safe mode, last known good config, start as normal, etc.) and it gave five lines of saying something about partitions and such and then stopped.  But when trying to load from the CDROM (trying to load a HD tools disc), I get the PCI listing screen, and a few lines of loading.  It says:
Boot from ATAPI CDROM
1.44 Diskette
Starting Caldera DR-DOS

Ext.  Mem Manager 1.05
copyright etc.
XMS Manager installed.

It quits there.  So that leads me to believe it wasn't the HD, because obviously the CDROM should be able to load up.  So then it puts me at one of three things.  RAM, CPU, Mobo.  What's the most likely candidate?  I have a few things to swap out, but I'm not one for trial and error if I can save a few minutes with someone's assistance.  (In the meantime, I'll be doing other things.) 

Also, I can build my own PC and such, install OS.  However, I'm not sure how to tell what is wrong when something decides to fail on me, as in this case.  Is there a website that lists how to tell (in general, at least) what may be the problem without having to go through a bunch of trial and error?

[hypernova]

OK...what the hell.  Just went to remove the CD I left in there, and was on the OS method screen.  So I chose Last known good config option.

And it loaded.  So what just happened?

[AtomSmasher]

The power supply is the most common component on a computer to break and it could be giving out weird voltages causing the computer to act erratically.

It's also possible it was caused by static electricity which dissipated when the computer was shut down for a while.  Although I've never actually seen static lock on a computer without physically touching one of the internal components, only heard that it could happen, so it's possible thats just an old nerds tale.

[Daniel270]

[testicle187]

OK...what the hell.  Just went to remove the CD I left in there, and was on the OS method screen.  So I chose Last known good config option.

And it loaded.  So what just happened?

Had you installed any new hardware recently or shut it down incorrectly?  Normally when Last known good config fixes something its because a new piece of hardware or new drivers were installed.  Could also be because of new software?  Hard to tell without more info...

Also, was it blue screening, resetting, or freezing when it wouldn't load?