This assumes that info is being typed in cell A1:
Private Sub Worksheet_Change(ByVal Target As Range)
Set a = Range("A1")
Set t = Target
Set b = Range("B1")
If Intersect(t, a) Is Nothing Then Exit Sub
If a.Value <> 1 Then Exit Sub
x = Application.InputBox(Prompt:="Yes/No", Type:=2)
Application.EnableEvents = False
If x = "Yes" Then b.Value = 1
If x = "No" Then b.Value = 2
If x = False Then b.Value = 0
Application.EnableEvents = True
End Sub
Because it is worksheet event code, it is very easy to install and use:
1. right-click the tab name near the bottom of the window
2. select View Code - this brings up a VBE window
3. paste the stuff in and close the VBE window
If you save the workbook, the macro will be saved with it.
To remove the macro:
1. bring up the VBE windows as above
2. clear the code out
3. close the VBE window
To learn more about macros in general, see:
http://www.mvps.org/dmcritchie/excel/getstarted.htm
To learn more about Event Macros (worksheet code), see:
http://www.mvps.org/dmcritchie/excel/event.htm