Troubleshooting : Find when system hangs ?

I am really newbie in SysInternals.

Since a few time, a computer using still XP SP3, which is used as a little server (with a motherboard designed for) after a while (some hours, sometimes two days) stops working.

Exactly like disk errors, processors stopping (heat) etc. not any control, no blue screen.

Disks have been scanned, memory checks etc. Nothing found

To try to find the problem, I think that a system activity (processes, disk accesses, network) which will trace on disk (not buffered) will stop writing when system will stop.

So I could read back the state before failure.

May be I will find something which happens repeatedly.

Note (subsidiary question) : I have on this system a "watchdog". To implement it I would need to survey all running processes and services and stop updating the timer (remaining time before reboot, at defined system address, I have the assembler routine). If the hang doesn't come from a processor hang, (but for example 100% processor used by priority task...) the system will reboot automatically. I don't know if any standard soft is able to do it.