OK, so yesterday I was using my old 1997 166MHz computer. I'd been using it, left it for a couple of hours, so it'd gone into standby. But when I tried to bring it out of standby, the screen was just black withg a flickering cursor in the top left corner. I tried Ctrl + Alt + Del and everyhing but that didn't work. So I turned it off, then turned it back on again. But now when it starts up the first thing it does is do a memory test.
It like goes thru the emmory up to about 32,000 KB or something, then does that twice more (its says OK next to it the whole time), then just stops. Usually it then checks for the CD-RoM being present etc.
I can't press Del to enter Setup, if I do, the numbers just go to 32,000KB and stop str8 away.

Sounds like a flat mainboard battery...

Battery sounds like a reasonable place to start to me too, if you aren't familiar with what's "under the hood" you'll find a picture of a typical motherboard battery (CMOS Battery) In This Thread (http://www.pcguide.com/vb/showthread.php?s=&threadid=25502) Batteries are about $3 or so at most department stores, a bit more at drug stores or computer shops.

WHile you have the cover off, clean the dust out, canned compressed air and a make up brush works well. Be sure and get the heatsink and fan as clean as you can.

Then check all cable connectins, reseat RAM and video card. Let us know if you still have problems at that point.

My battery went years ago, but all that meant was that the clock reset itself to 01/01/96. I put a new battery in, which was the same volatge, but a different shape. It was a new battery, but maybe that's made it go wrong.
It is quite dusty inside. I'll try those things.

I've dusted thoroughly, put a new battery in, checked for any damage or disconnected wires, but its still not working.
I've got important stuff on that computers HD so is there a way I can access it.

Your hard drive is probably fine. However, you will have to get your computer running or place the drive in a working PC in order to access its data.

The problem is definitely hardware related and could be to a failure of virtually any component (drive, PSU, motherboard, video card or other card, CPU, or memory). Some diagnostic work is indicated as outlined below:

Bare bones boot to narrow search for malfunctioning hardware device:

Test takes only a few minutes. It sounds difficult, but in practice is quite easy. Take notes if necessary on the devices you disconnect so that you will have no difficulty putting everything back together:

1) When you open the case, make sure that all fans are working, that the heatsinks are not clogged with dust, and that the motherboard appears to be normal – no bulging, leaking or exploded capacitors.

2) Disconnect power and data connectors from all drives (CD, DVD, HD, floppy) leaving only your boot hard drive. Remove any cards from the expansion slots except your video card. Disconnect any devices attached to serial, parallel, firewire or USB ports

2) Try to boot. If the problem has gone away, add devices back one at a time until you discover the defective component.

3) If the problem still exists, you have narrowed your search to motherboard, memory, CPU, hard drive, video card and power supply.

Further testing:

1) Try a different video card if possible.
2) If machine won’t boot, remove one memory stick then the other if you have two, or try another compatible memory stick.
3) Look at PSU voltages with voltmeter – (red is 5 volts, yellow is twelve volts, black is ground). Or enter bios setup and read the voltages. If machine won’t boot try different PSU.

I removed the sound card, the USB card, the modem. I disconnected the power supllies to the CD-ROM, CD-RW, Floppy Disk. I removed all the peripherals also.
I then removed both SIMMs, dusted around where they go in and such, replaced them. I removed the video card and put it in a different slot.
I removed the fan, removed all the dust, removed the CPU, checked it for damage and replaced it.
And it's still not wokring.

I can't decide whether it completes the memory test or not. Usually it tests the memory three times (up to about 32,000KB) then checks for the Hard disk and CD-Rom and stuff, but it seems to finnish the memory test and freeze.

Also, this might be importnant. When I press DEL to enter BIOS setup, the memory test automatically jumps to 32,000KB and freezes.

This sounds like a motherboard problem to me, maybe a corrupted bios.

If you have an old video card you could try that. And you probably need to measure voltages to make sure your PSU is working.

It wouldn't hurt to disconnect your hard drive and try to boot with a StartUp floppy or some other dos boot disk, just to see if it will boot. That would eliminate some other things like memory, CPU, video, PSU from the list of possible bad components.

Disconnect both ribbon cables going to the IDE controller and see if it will boot until it tells you it finds no boot device or asks for a floppy. Also go into BIOS and look in Advanced CMOS settings for "Quick Power on Self Test". That should stop the 3x memory count and only check it once. Depending on your BIOS it may be worded a bit bifferently, but should be similar.

My guess is the CD ROM is causing trouble, sometimes they will go bad and stop the POST or you'll get no video at all. Don't really know why it happens, but it's caused me no end of headaches in the past on older systems. A bad ribbon cable could do the same I think, or it mat be just a bad connection. Reseating them a few times might help. Same for RAM and video card.

Yeah, I missed the fact that during your bare bones boot you just disconnected the power from the drives. The drive should be completely disconnected (power and data) as Paleo Pete pointed out.

OK, I'll try that. Thanks guys, for all yoos helpage

Have you checked your CMOS yet? It is possible that your COMS values had been set due to the bad battery (or maybe as a result of powering it down) and you need to change it.