Thank you jpetrie for telling me about the extra debug information.

When I turned the debug information on I noticed that I had allocated a few D3D resources in another thread by mistake. I added the code to allocate resources a long time after I had written the code to create the thread and had forgotten that this loading code would have been done in another one.

This explains why the error was common on the fast machine, it has a Duel Core CPU and my laptop doesn't. I guess there is some race condition in the resource allocator that causes this problem.

I moved the allocation code back to the main thread and now it all works fine.

Thanks Guys