HI guys, thank you all for your replies & suggestions.
I tried renderring out this morning with the case open and a large room fan blowing into it - it got further along in the encoding process but still crashed with the above error message. Whilst fiddling around in an attempt to start the process of reinstalling my OS, I changed a setting in the bios to run the pc in "normal" mode (balanced between power saving and performance) rather than all out performance. Well since then, I have managed to render out two 45 minute clips in mpeg2-dvd with the bit rate cranked up a bit, mp3 audio, etc.
So the only thing I figure is that it's a hardware issue, ie cpu running too hot (despite the fan thing not working but furthering the encoding), but at this stage I have been able to encode a couple of videos albeit a little bit slower.
Thanks,
Stevan