Combo values depends upon another field?

[edster]

I have a serial no combo box which displays serial numbers available for shipment. What i need is some way to make the combo box show the values 'serial numbers in stock' (simple sql) when the [tocompany] is not 'CVP' and to show 'serial numbers out' (another simple query) when the [tocompany] is 'CVP'

Basically i need to change the row source depending on the value in the [tocompany] field.

Thanks in advance

Ed