Display corrupted

Hi, I wonder if anyone can help me with a problem that is driving me mad. I am writing an ActiveX control using MFC and it is all coming along well except that whenever I run the control, at some indeterminate point afterwards usually when the control is not even running my display becomes corrupted; menus and window contents appear in the wrong place fonts get mixed up and the task bar starts drawing at the top of the screen. There is no evidence of what might be going on when the control is actually running as in all other respects it works fine.
Any suggestions?
Rupert