Creating a dropdown list

Hi, I need help in Excel. I'm currently using Excel 2007. My problem is this:

I want to create a dropdown list that is dependent on 2 variables. I already got the vlookup part, but my problem is in the creation of a dropdown list. In the snapshot below/spreadsheet attached, I have 6 columns depicting pipe sizes (column A) and pipe schedules 10, 30, 40, 80, and 100 (columns B to F). For every diameter (A4 to A24) there is a corresponding thickness for every pipe schedule (B4 to F24). I created a dropdown list in cell B26 specifically for the diameter and cell B27 for the pipe schedule. The thickness (cell B28) is the result/intersection of the two previous cells. I want to create a dropdown list in cell B27 that will ignore zero/null values in columns B to F, so that the user will be limited to the choices which have a positive value. For ex., for the Diameter in cell B26 I will select 150, the choices for Schedule in cell B27 will only list down values 40 and 80 because they are the only ones with corresponding thickness, the other columns will return 0 values.

How do I do this? Is this possible with Data Validation tool?

Thanks in advance.