Use the mousedown event of the listbox, and check for the mouse button. If it's the right button then count the selected items and set the contextmenustrip arcordingly
vb Code:
  1. Private Sub ListBox1_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles ListBox1.MouseDown
  2.         If e.Button = Windows.Forms.MouseButtons.Right Then
  3.             If ListBox1.SelectedIndices.Count > 0 Then
  4.                 ListBox1.ContextMenuStrip = Me.ContextMenuStrip1
  5.             Else
  6.                 ListBox1.ContextMenuStrip = Nothing
  7.             End If
  8.         End If
  9.     End Sub