Private Declare Function SetWindowLong _
 Lib "user32" Alias "SetWindowLongA" ( _
 ByVal hWnd As Long, _
 ByVal nIndex As Long, _
 ByVal dwNewLong As Long) As Long

Private Declare Function CallWindowProc _
 Lib "user32" Alias "CallWindowProcA" ( _
 ByVal lpPrevWndFunc As Long, _
 ByVal hWnd As Long, _
 ByVal Msg As Long, _
 ByVal wParam As Long, _
 ByVal lParam As Long) As Long

Private Declare Function RegisterHotKey _
 Lib "user32.dll" ( _
 ByVal hWnd As Long, _
 ByVal id As Long, _
 ByVal fsModifiers As Long, _
 ByVal vk As Long) As Long

Private Const WM_HOTKEY As Long = &H312
Private Const VK_SNAPSHOT As Long = &H2C
Private Const GWL_WNDPROC = (-4)
Private Const WM_DESTROY = &H2&
Private Const MOD_ALT As Long = &H1

Private hHandle As Long
Private hPrevWndProc As Long

Public Sub HookPrtSc(frmHandle As Long)
 hHandle = frmHandle
 'Register both Alt+PrtSc and only PrtSc as hotkeys
 RegisterHotKey hHandle, 1, MOD_ALT, VK_SNAPSHOT
 RegisterHotKey hHandle, 2, 0, VK_SNAPSHOT
 hPrevWndProc = SetWindowLong(hHandle, GWL_WNDPROC, AddressOf WinProc)
End Sub

Public Sub Unhook()
 SetWindowLong hHandle, GWL_WNDPROC, hPrevWndProc
End Sub

Private Function WinProc(ByVal hWnd As Long, ByVal uMsg As Long, _
 ByVal wParam As Long, ByVal lParam As Long) As Long
 Dim sFile As String
 Dim nSize As Long
 Select Case uMsg
 Case WM_HOTKEY
 WinProc = 1
 Exit Function
 Case WM_DESTROY
 Call Unhook
 End Select
 WinProc = CallWindowProc(hPrevWndProc, hWnd, uMsg, wParam, lParam)
End Function 

Private Sub Form_Load()
 HookPrtSc Me.hWnd
End Sub

Private Sub Form_Unload(Cancel As Integer)
 Unhook
End Sub