I have been looking to see if anyone has the same problem, i found some close but well a few variances. My computer is a Pentium 4, windows xp home, with 120 gig hd and 512 memory. When I start it, it will POWER on the fans and everything like it would normally then just stops. The fans still run and the lights are on but no power to hard drive, cd drive or monitor. If I boot it with a cd or floppy it will work while running either of those THINGS but once through it stops, with fans on and lights on. I had it set to do automatic updates with microsoft, and well it tried to do service pack 3 update and for some reason never able to install until the other day. Now this is happening. I did do the recovery cd that came with it but once the cd stopped the computer did the same thing. So it never finished recovering the os and programs. I got half way through with it before it quite. Any HELP would be greatly greatly appreciated!!!!!!

Do you get anything on the screen while it is booting before it just stops? Does it always stop at the same point?Yes, it stops SOMETIMES right after the windows logo appears and sometimes it doesnt get to that. The other day it stayed on for about an hour I believe but now just that little bit. Like it is teasing me. You may have to save all the data you want to save, and then completely re-install Windows and the service packs. (Not much help, I know, but it seems like the operating system has become corrupted.)I have tried that and the minute the recovery cd stops the computer shuts off. If I am running a bootable cd or floppy it will stay on until that is done. very weird huh? Little same what happen to my pc huhh always rebooting in random time... so I format the HDD to the other pc but it getting worst.... now i face another problem can't boot my cd-rom.. hehehe...I wonder if it is a problem with the hard drive, itself?It may be hard drive itself, RAM, or power supply problem.
Run memtest: http://www.techspot.com/vb/topic62524.html
Run HD diagnostic: http://www.tacktech.com/display.cfm?ttid=287