You could try this:

Place this behind your Sheet, it will update evrytime the value changes.

You will have to enter however many additional arguments you want based on
how many lines across the sheet you want to go.

HTH
Mick.

Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("A1")) Is Nothing Then
Select Case True
Case Target.Value = 1
Rows("1:1").Borders(xlEdgeBottom).LineStyle = xlNone
With Range("B1:C1").Borders(xlEdgeBottom)
.LineStyle = xlContinuous
.ColorIndex = 0
.TintAndShade = 0
.Weight = xlThin
End With
Case Target.Value = 2
Rows("1:1").Borders(xlEdgeBottom).LineStyle = xlNone
With Range("B1:D1").Borders(xlEdgeBottom)
.LineStyle = xlContinuous
.ColorIndex = 0
.TintAndShade = 0
.Weight = xlThin
End With
End Select
End If
End Sub