Drop down boxes in excel 07

Dear All,

I have two columns A & B which i wish to turn into two drop down boxes, the first drop down box i wish to search column B for 1 of 4 allocations (allocations to the cell directly opposite to column B). I then wish the second drop down box to return a list of names which are allocated to the 1-4 allocations.

e.g.

Column A Column B

Bob17 1
Bob11 1
Bob70 3
Bob4 4

The first drop down box will have a selection of 1-4 then depending on which is selected it will return the names in column A into the second drop down box, which are allocated to the number. E.g. if 1 is selected in the first drop down the second drop box will show the user both the names of Bob90 and Bob11 which are allocated to that number.

I currently know how to create a single drop down box using Data Validations but am unable to make the second box to look at the returns of the first.

I very much appreciate anybodys help.