So, the index of the shortest string?

There are ways to do it that use less code, but they are probably slower than the old fashioned way:

Code:
dim minIndex as integer
dim minLength as integer = Integer.Max

For x = 0 to listbox.Items.Count -1
 if listbox.Items(x).Length < minLength Then
  minLength = listbox.Items(x).Length
  minindex=x
 End If
Next