No, I'm talking about the event log in the OS - what version of Windows is it?
(For Windows 2000 onwards, right-click My Computer, click Manage and go to the Event Viewer.)
If the system is doing a spontaneous restart, not even a blue screen, then it's not likely to get chance to enter anything in there though.
If the problem appears to be geared around the graphics card entering certain modes (MPEG decryption, Direct3D, whatever) then that is also a good suspect.
Has this been working fine until one day it decided to start doing this, without any software or drivers being updated just beforehand?
If so then it's sounding like a hardware issue.
I would check the CPU temperature anyway, just to be able to rule it out.
Motherboard Monitor Lite is freeware which can show you the current temps and fan speeds, so long as your motherboard has the relevant chipset.
http://mbm.livewiredev.com/download.html