I have managed to get the AVI file to run by renaming it to a MPG file without any other changes and the MP4 file also runs OK however the AVI file runs at tremendous speed. I ran dxdiag which showed Directx 11 was installed, I also ran repair on PowerPoint which showed no problems. It's all a bit of a mystery but all's well that ends well. The tip about renaming I found during searches for "AVI won't run in PowerPoint 2007"
Thanks for your help
Albie