Crashes with no data

My system has been crashing every once in a while over the last two
weeks. No data in the system log before a crash (and reboot) so it's
obviously hardware related. No blue screens. Always reboots
immediately. Checked Allendale temp at 38 deg C. Thought temp might be
the problem because machine has typically been running at least an
hour. It was up against a bookcase on one side so I moved it out from
the wall to expose the vent hole and fan on that side. Temp went down
to 34 deg C. It didn't crash for a week, then today it crashed again.
Not seeing a relation between software applications and crashes though
this last time composing e-mail, the machine hesitated before catching
up with typing then after the e-mail was comlete and sent it crashed -
may be unrelated.. This is a 5 month old machine. ASUS P5N-E SLI mobo
with ASUS EN6200 LE video card and Intel Allendale CPU, 2G RAM.

I'm not sure what to troubleshoot first or how to go about it - cpu,
mobo, PSU, hard drive, memory? Any ideas? What it isn't? What it could
be?

BCWD