Gateert,

Thank you for going to the trouble of describing the solution to this problem.  I spent most of an evening and the next day trying to get GUIMiner working.  Your summary of using Driver Fusion (the free version) to clean up the drivers and installing the beta driver version in your post worked for me as well.

The 6970 is running around 380 mh/s.

Your post was very much appreciated.