Thanks for your reply. All the PCs have had the same AU's installed and
are all basically the same spec (Dell Optiplex's of roughly the same
age). If it is that how do I work out which driver is causing the
problem from the memory dump? We never download any driver updates from
Microsoft - only ever from the hardware manufacturers, and we haven't
done that on any of the machines in the timeframe where we have seen
things go wrong.

Iain