Not sure about this...what OS are you running Iz?
I have seen a possibly related issue on my desktop. Very occasionally MAMEWAH will not exit properly and whatever is 'behind' MW does not get refreshed properly (display-wise). When I go into Task Manager it seems MW is still running although not visible, and not responding. End Task and all is ok again...
As I say this is very occasional for me, so I haven't been able to trace a problem. Also not sure if it is relevant but I have always had Explorer running when this has happened. This has only ever happened since I moved sound/input etc. to DirectX, so it could be an issue with that.