Ran the memory diagnostics tool - no problems detected.
Ran the Vista Install DVD Startup Repair - went through in seconds finishing with 'Startup Repair could not detect a problem'
So all looks well, but there is definitely still a problem; the PC has frozen twice today already, once on re-initiating from 'Sleep' and once when trying to check mail on a user account - both times required the PC to be powered off and then on again.
Clutching at straws here,but I don't suppose this could be mouse related? The freezing 'appears' to be happening when moving the mouse? (DELL USB Optical)?