Conditional Formatting based on another Cell

I have an issue with Excel 2007 and I wonder can you help.

I have a Column A that you can select a “Yes” or “No” for each cell which I have set up under Data Validation.
Based on this if I select “Yes” in cell A1 it should have no effect on cell B1.
Based on this if I select “No” it has the effect that cell B1 is populated automatically with a Zero.

Can you assist??? As I cannot get this to work correctly...