XP-P4 Computer boots into reboot loop

My wife's computer [xp; P4 2.8 Ghz, 478 pin cpu; 512 MB RAM] died the other day. It had been working flawlessly for years. Basically when she turned it on nothing happened though it had been working fine the evening before. After a few more tries it started going into a boot up loop: it starts to power up but only for a second or two and then it dies and it starts to power up again, dies and repeats a few times and eventually the loop will just stop though sometimes it will resume looping after a delay which suggests a CPU cooling problem. I find it a little hard to believe that the CPU could heat up enough in two seconds to require an emergency shutdown, however, stranger things have probably happened.

I tried replacing the power source but the same behavior occurred so I figured it was probably a motherboard or CPU problem though there could be other causes. After looking at options I decided the most cost-effective thing to do would be to buy a used but working replacement motherboard/CPU combo on eBay. They can be had for under $50. Well that arrived, I plunked it in and -- voilà -- the same old looping again.

Next I decided to just verify that her bootable hard drive was indeed working so I popped it into my computer [a similar configuration] and -- lo and behold -- it wasn't. It would boot into a screen suggesting a boot into safe mode but no matter how I booted it only reached a black screen with something like a blinking cursor in the top left corner.

Thinking her boot disk was hooped I put it on my computer as a slave to take a look and things seemed fine. Running short of ideas, I deleted the partition and reinstalled Windows XP. She keeps all of her data on a separate hard disk so that's secure. I had suspected the bootable hard drive was toast but actually it was fine so it now seemed like it had been an OS problem. No big deal, or so I thought. Upon returning the drive to her computer we were presented with the same old, same old looping behavior. Now I love a good mystery but this is ridiculous.

Now I became suspicious of the RAM so I switched the RAM from my computer to hers but no change. Next I switched out the video card. Nada. I tried two more power supplies. Nada. Nada. I tried reconnecting all of the various wires to the new [used] motherboard to no avail. Grasping at straws I tried two different power cords and even tried bypassing the power bar and plugging the power supply directly into the wall socket. Nada. Nada. Nada.

What's left? Just the box. Is there some remote possibility that the on/off switch has worn out and could cause this kind of looping? Yes I'm stretching but as Sherlock might say: "When you have eliminated the possible, whatever remains, however improbable, must be the truth."

To sum up I've tried:

A fresh motherboard -- supposedly in working order
:confused A fresh CPU -- supposedly in working order
:mad Four different power supplies -- one of which, was definitely in working order
:tired Fresh RAM -- and definitely in working order
:cry Fresh OS reinstall -- hard drive definitely in working order
:major Three different power cords -- haven't verified but, come on...
:-o Circumventing the power bar
:banghead Checked and rechecked all connections

Am I going loopy? What have I missed? Yes there's a million to one chance that both motherboards are hooped; or both CPUs but I really don't feel like potentially ruining my own computer to find out. Any other suggestions?

 

Take out the cmos battery and let it clear

OR

Try updating the BIOS from the manufacturer, its possibly corrupt.

 

Hi,

You may have already considered this but, I have been getting lots of comps lately with the CPU heatsink completely clogged with dirt. Take a good look. You can usually see the dirt by looking through the blades of the fan. Maybe?

Good Luck, Jim

 

Thanks Guys:

jlphlp: Yes the heatsink was fairly gunky though other parts of the computer were a real horror show. That might have contributed to the original MOBO/CPU failure if that's indeed what happened.

MoPaR: I tried resetting the C-MOS a couple times but to no avail.

I've pretty much concluded that RAM issues are preventing the replacement MOBO/CPU from firing up. By switching out various DIMMs I have on hand I have managed to get beyond POST and into the BIOS and once even got to the XP splash screen.

Most are incompatible, however and even the best DIMM produces a "RAM R/W Test Failed" error. The replacement board seems to be particularly finicky so I've ordered DIMMs that should work.

I'm fairly confident that this is the fix: Original MOBO/CPU hooped; Replacement MOBO/CPU incompatible with existing RAM. I'll try to come back and verify if I don't forget.