Random crash and reboot (but it shouldnt reboot?)

Hey guys,
first of all thanks for taking the time to read this -- I'm having quite an annoying problem and I'm running out of ideas.

My PC crashes every now and then, sometimes once a week, sometimes 5 times in an afternoon. I noticed I could recreate the crash by copying a large file (2Gb) to my USB drive so I thought it was USB related, but a few days later it crashed while I had nothing in the USB drive. However, I do believe I was running 4 or 5 programs, MAtLab I believe, and maybe also a video converter.

Anyway I ran Prime95 for the longest time with no crash, also ran Memtest 86 (I think that's what it's called) also with no errors.

I have an AMD Athlon 3800+ x2 with 3Gb Ram and XP SP2.

Now the funniest thing is I did unticked "Automatically reboot" in the system's advanced tab, but when it crashes, it freezes for about 8-10seconds and immediately reboots (even though it shouldnt) and there is no minidump.

So what's left? The Power supply? Any other ideas? Why does it still reboot when crashing - I don't get that one.


Thanks so much.

I went on the ECS website to look for possible updated drivers, and I found a new version of the Bios.
Here's one of the things it fixed:

Fixed Use USB storage large files size copying to HDD or USB HDD failed issue

lol didn't update yet but it should do it. I'm only posting back so that if people have similar problems, obviously check driver updates but don't forget Bios updates too!!