Try this:

Give frmMain a special tag (like 25360) and use:

VB Code:
  1. Private Sub cmdBack_Click()

  2.     frmMain.Show

  3.     

  4.     Dim frm As Form

  5.     

  6.     For Each frm In Forms

  7.         If frm.tag <> 25360 Then

  8.             Unload frm

  9.             Set frm = Nothing

  10.         End If

  11.     Next frm

  12. End Sub