Bon...
Je n'en vois pas l'utilité (le faire en mode exécution ? et pas en mode création ? pourquoi ?..), mais si la chose te plait tant :
Private Declare Function GetWindowLong Lib "user32" Alias "GetWindowLongA" _
(ByVal hWnd As Long, ByVal nIndex As Long) As Long
Private Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" _
(ByVal hWnd As Long, ByVal nIndex As Long, ByVal dwNewLong As Long) As Long
Private Const GWL_EXSTYLE = (-20)
Private Const WS_EX_APPWINDOW = &H40000
Private Sub Form_Load()
SetWindowLong Me.hWnd, GWL_EXSTYLE, (GetWindowLong(hWnd, GWL_EXSTYLE) And Not WS_EX_APPWINDOW)
End Sub