For the last few weeks my PC (pretty old P4 with WinXP SP3) has been crashing randomly. It just suddenly reboots instanteneously. It feels hardware-related but I wasn't able to determine wether it's software or hardware that causes it. I did notice a pattern though: it's more likely to crash when I copy a lot of files over network or have uTorrent running, but sometimes it crashes when I am not doing anything with it.

Copying files from it over network causes it to crash in 1 to 10 minutes almost every time. Using torrents causes it to crash every 1-3 hours. With neither or that on it crashes every 24 hours or so.

I ruled out the following probable causes:

• PSU (I bought a new one and turned off most of the drives so the power is sufficient 100%)
• Bad HDD or interface cable on my SATA disk from which I was originally copying the data over network (bought new SATA cable and later yanked out the HDD completely, PC still crashes without it)
• Video adapter (AGP slot is now empty, using the onboard VGA at the moment)
• Network adapter (removed it from PCI, using onboard LAN)
• CPU (I think: I changed the termopaste and it's temperature is below 50C)
• RAM (I think: I ran Memtest86 and it didn't show any errors)

At the moment I only have only one system HDD and DVD drives, a mouse and a keyboard plugged in.

The fact that it crashes most often when I use network extensively makes me think that maybe it's software related (I removed the network adapter from PCI and now am using an onboard one, so network hardware is unlikely to cause problems). I am now pondering system reinstall but it's not a pleasant solution so I decided to ask wether there are better ideas first. If someone can share a good diagnostic tool it would be great because I didn't find anything good.

Thanks in advance, I hope that "help to diagnose" questions aren't entirely banned here.

EDIT: Motherboard is actually ~4 years old as I replaced it back in 2007