XP won't boot for my father-in-law (but will for me, seriously)

I built my father-in-laws PC several years ago. From old spare parts I had leftover.

Specs:
Mobo: Intel D865GLC
Gateway MIDWAY 4000888 socket 478
CPU: Intel P4 3.06ghz 800fsb
Video: Radeon 9200 AGP 8x (128MB) DDR 64-bit, 3DAccelerator
Audo: SIIG DP SoundWave 4 Channel PCI
Ram: 2gig (PC3200 DDR SDRAM)
HDD: 160gig status:healthy, 96gb free space
CD drive: LITE-ON CD-ROM LTN-5291S
DVD drive: LITE-ON DVDRW LH-20A1P [CD-ROM drive]
XP Home edition, SP3

OK, this PC has been running fine since I built it, I did add a better video card last year so he could run some slot-machine games. The only other thing that changed is he had U-Verse installed several months ago, but the PC was running fine until just a couple weeks ago.

He said he was getting an error on bootup and wrote it down:
Reboot and select proper boot device or insert boot media in selected boot device.


So, I agreed to take a look at it.
He brought it over and I hooked it up to my KVM switch and it booted up just fine. (I didn't change anything!!)
I shut it down and rebooted it almost a dozen times, no problems, none. Booted every time.
My wife even shut it down and rebooted it. She asked me how I fixed it and I said I didn't, it wasn't broke to start.

She took it back to him, he hooked it up and it refused to boot AGAIN.

So he brought it back, I hooked it up to my KVM again and it boot just fine. WTH????

IF I understand the error message, it's trying to boot from something else, BUT WHAT??
I'm going to try going to his place and only plugging in the Monitor, Keyboard, and Mouse.
Then see what else he's plugging in, but it doesn't make any sense to me.

Any suggestions?

What power supply is it running and what's the wattage? Could be it's not getting enough power to boot properly. Check which drive is first boot drive, also.

If you're using your keyboard and mouse, and monitor, if the psu doesn't have enough power it could be causing the problems.

Or his electrical voltage is not high enough.

He should try running an extension cord from another wall socket.

Have him disconnect all USB devices and then try booting it.

I'm going to take it back to his house and only connect the keyboard, mouse, and monitor and try to reboot.
Then add his other connections (Internet, USB, etc.)


This is really weird because it's been working for years and works perfectly fine at my house on my Monitor, Keyboard, and Mouse.
I'll also try connecting to another socket and check what PSU and wattage are.

From the error message, it sounds like the computer is attempting to boot from something other than the hard drive and cannot boot from it

For example, let's say the BIOS is set to try to boot from some USB device _before_ trying to boot from the hard drive, If so, and he has say:

a USB key (flash drive | thumb drive) plugged into a USB port or hub,
or a SD card plugged into a multi-reader or a USB printer,
or a CD/DVD in an external USB CD/DVD drive,
etc., etc., etc.,

then the computer may be trying to boot from them instead of the hard drive.

Weird, Weird, Weird.

I brought his PC back to my house and rebooted it at least 15 times, worked perfectly every time. I also ran several anti-virus, anti-malware, anti-spyware, spaces cleaners, and defraggler. It still booted perfectly every time. Then I went into the BIOS and changed the boot sequence to ONLY boot to the HDD, nothing else. I successfully rebooted, then I ghosted each drive to a backup drive.

I took the PC back to my father-in-laws, plugged in the mouse, keyboard, and monitor only, nothing else (no internet, printer, or sound/speakers). First time I booted it up.... FAIL! Same message he was receiving before. Reboot and select proper boot device or insert boot media in selected boot device.

So I unplugged the mouse and keyboard, the only thing plugged in was the monitor and power plug. That's it! It gave me an warning message that there was no mouse and keyboard, then FAIL AGAIN! WTH!!??

SO, I thought maybe his surge protector or power in that room was bad, so we ran an extension cord from the kitchen. (I plugged the mouse and keyboard back in so I could control the PC). Rebooted and FAIL AGAIN. (this is really beginning to T me off!)
At this point I rebooted to the BIOS. I looked at the Boot sequence (I had changed it to ONLY boot to the hard-drive when I had it at my house), but the boot sequence now showed all drives, EXCEPT the HDD. There was no HDD even available to select!! So I went to another tab that showed all of the PATA and SATA ports and what was connected, but all of the PATA ports were empty! NO HDD in the list.
I shut it down and opened the case to check all of the connections. I removed every drive connection (CD, DVD, and HDD) and blew in them, then reconnected, but I swapped the HDD and CD drive ports. I rebooted to the BIOS and the HDD was listed. I rebooted and allowed it to finish.
IT BOOTED CORRECTLY TO WinXP!!!
I shut it down and switched the cables back (reverted the CD and HDD).
REBOOTED and it WORKED AGAIN!!!
Shut it down. Switched the power cable back to the Surge Protector and plugged in the Internet, printer, and speakers).
Rebooted it again, AND IT WORKED!!!!!!!!!

IT COMPLETELY baffles me that it:

• didn't work for him originally at his house
• worked for me perfectly every time
• then it didn't work at his house again, not once
• then it worked for me perfectly every time AGAIN
• give it back to him and it doesn't work AGAIN, until I switch the cables (but why did it work for me??)

I just don't get it, and not even confident that switching the cables was the fix.

JUST PLAIN WEIRD

any explanations? I'm dumb-founded! (but my father-in-law couldn't be happier that it works again!)

That is weird, really weird.

Same position (vertical|horizontal) at both locations?

Flaky power supply maybe?

Cracked solder joint or wire?

Failing capacitors?

Motherboard standoff(s)?

Faulty house ground?

...