Me.BorderStyle = 1 ' 2

  Dim aStyle As Long
  aStyle = GetWindowLong(Form1.hwnd, GWL_EXSTYLE)
  aStyle = aStyle And Not WS_THICKFRAME ' Or WS_THICKFRAME 
  Call SetWindowLong(Me.hwnd, GWL_EXSTYLE, aStyle)

'Конструктор
'BorderStyle=2 Sizable (или 1 - неважно)
'Caption = " " (пробел, но не пустой)
'ControlBox= False

'===============

Private Sub Form_Load()
  Me.BorderStyle = RND_CalcRND(1, 2)
  Me.Caption = "" 'пустой
End Sub

Public Function RND_CalcRND(lowerbound As Long, upperbound As Long) As Long
  Randomize
  RND_CalcRND = Int((upperbound - lowerbound + 1) * Rnd + lowerbound)
End Function

Private Sub Form_Load()
  If sys_WinVista And ThemesEnabled(Me.hwnd) Then
    Me.BorderStyle = 2
  Else
    Me.BorderStyle = 1
  End If
  Me.Caption = ""
  ...