Computer turns on for one second, but thats it.

Most likely the cause of your mobo failing or even your PSU (the power test is not definitive). If there's no POST beep code at all (just turns off) then it may very well be one of those two. Make sure your power button also isn't shorting somehow or the contact getting stuck.

Any ideas how I can be sure, short of buying new parts.

Also how can I tell what voltage my power unit is, sadly all the specs have been lost long ago.

Try a friends power supply or ask around for someone that has a spare if you can to eliminate the power supply. There are pins on the motherboard where the cable for your power button go. You can unplug the power button from the motherboard and short those two pins together (with a flathead screwdriver) to complete the circuit and the computer will turn on eliminating the power button. Here's a youtube video of someone doing it. The quality of the video is crappy but you can get the gist of what needs to be done if your unfamiliar.

YouTube - How to turn on a computer without switches or buttons


Good Luck

Also, I read on a forum a while back that you can use a voltmeter to test your power supply if you have access to one. (Could try googleing that) Also the specs for the power supply are usually a sticker on the side of the power supply when you take the side panel off your computer case.

Hope that helps.

Ok I found out my power unit is a largish 750 watt one. I assume that is mainly to power my graphics card which, when the computer was made, was quite a good one. Does that mean I could hook up any old lower watt power unit to my motherboard to test if the power unit works? Or will this just blow out anything that is left?

Thanks again

Ant

What's the brand and model of the 750W PSU and the brand and model of the video card?

To add to Mark's inquiry, what's the wattage on the lower watt psu? It won't "blow out" anything by having an insufficient power supply, but it will cause even greater instability.