vb6 in windows 7 ocx problem

Hi
We are working with VB 6 project in windows xp OS. It works fine. Now we have changed our OS to windows 7. After that we have created all the DLLs and OCXs of our project. Some of the OCX are not working now. Please tell me the reason. While we are trying to open that screen which refers the ocx, we are getting the error 'object or with block variable not set." Please guide me to solve this problem.

Thanks & Regards,
GMKK