This is quite a long post. It needs some explaining to avoid wasting your time.

My other half has a HP zd7000 series laptop, more precisely the zd7143ea. It's a second hand machine but is in good condition. Windows XP Home is installed on it and it was given a 1GB RAM upgrade so it has 1.25GB of RAM instead of the standard 512MB fit. It also has a new WD hard disk. All software, drivers and BIOS are up-to-date. External peripherals include HP F4580 Printer, Logitech wireless keyboard/mouse, WD Elements 1GB HDD.

The problem is (as the title suggests) that the machine spontaneously powers off without warning or completing the normal process. It's like someone has whipped out the power lead without the battery installed. The fault is intermittent, but when it happens once it will happen again quite soon after. Once it's powered off, it takes a bit of effort to get the machine to start again.

She's not using the machine for anything intensive, just basic office type tasks. The shutdowns happen normally when she's using MS Office, although they don't occur exclusively with MS Office. Office 2007 was installed, but in an effort to test the issue, I uninstalled it and installed Office 2003 instead. The shutdowns seem to have reduced VERY slightly.

I've tested the machine with memtest with no faults. I've had the back off to check for damage or loose connections - there are none. I used a burn-in program for a few hours while monitoring the CPU and GPU temps and they didn't go particularly high and the machine didn't crash, even when I covered the vents to push it. I think the highest recorded CPU temp was 55C which is well within the specs for a P4. The fans work fine and are pretty clean.

From my searching, I have found the following suggestions (they haven't worked):

- Dodgy battery (tested with battery removed)
- Dodgy LAN (turned it off and used wireless)
- Dodgy wireless (turned it off and used LAN)
- Heat (no problem, see testing above. Normally lives on a glass desk too.)
- Disconnect external peripherals (done)

While I was checking through the forums, I found http://forums.windrivers.com/showthread.php?t=76534 so I'm going to try looking for that. I don't have access to a multimeter at the moment to check the power supply. Testing on battery isn't really an option as it won't last longer than about 40 minutes. I'm also wondering about the merits of removing the CPU, cleaning the surface and reapplying thermal paste, although I think I've established that heat isn't a factor.

Any suggestions??

My guess is something to do with the power supply, the on switch or a busted motherboard.

Thanks