Boot-up Problem - 26 seconds and it's dead!

[road-runner]

I know there are some seriously clever guys out there so please have a think about this one!

Basically, my PC (a 3 year old emachines 270, 1.7GHz, 40GB hdd etc) starts it's boot cycle but within 30 seconds it shuts down. The PS unit is still working but the CD/DRive/Floppy/CPU fan etc shut down - i.e. no power. Second time round, the system often works long enough to get into safe mode - sometimes even long enough to open up windows etc but in every case, after a few seconds to a few minutes (it varies) power is lost from the peripherals etc. Again, the PSU stays working - or at least the fan is rotating. And yes, I have changed the PSU - no improvement.

So, it is drivers? The boot sequence sound normal - disks running up etc. Or is it over heating of the CPU? If it's dirvers - why does it stay on long enough to get into windows sometimes. I also thought CMOS problems - battery etc but the motherborad doesn't seem to have a CMOS battery - at least it's not at all obvious (unless it's on the back side of ther board).

Any ideas, things to try gratefully accepted. It's got some good data I would like to recover.

Cheers and regards
Dave