HOWTO: Block CTRL+ALT+DEL and ALT+TAB in Windows 95

ID: Q161133

The information in this article applies to:

Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0
Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 32-bit only, for Windows, version 4.0

WARNING: ANY USE OF THE CODE PROVIDED IN THIS ARTICLE IS AT YOUR OWN RISK. Microsoft provides this code "as is" without warranty of any kind, either express or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose.

SUMMARY

You can prevent the CTRL+ALT+DEL and ALT+TAB key combinations from bringing up a task list in Windows 95 by calling the SystemParametersInfo function in the Win32 API. A step-by-step example of this appears below.

NOTE: This is possible only in Windows 95.

MORE INFORMATION

Sometimes it is necessary for a program to prevent the use of the CTRL+ALT+DEL key combination to bring up the Close Program task list to end a task or shut down Windows 95 and to prevent the use of the ALT+TAB key combination to switch tasks. The following technique uses the SystemParametersInfo API to trick Windows 95 into thinking that a screen saver is running. As a side effect, CTRL+ALT+DEL and ALT+TAB are disabled.

The Win32 SDK states:

"SPI_SCREENSAVERRUNNING Windows 95: Used internally; applications should not use this flag. Windows NT: Not supported."

Note that disabling CTRL+ALT+DEL is not recommended because the Close Program dialog box was created to enable users to terminate misbehaving applications. If a program "hangs" while CTRL+ALT+DEL is disabled, it may not be possible to terminate it by any method other than rebooting the machine, which could result in the loss of data. Also, this technique may not work in future versions of Windows.

Step-by-Step Example

Start a new Standard EXE project. Form1 is created by default.

Add two CommandButton controls (Command1 and Command2) to Form1.

Add the following code to Form1's General Declarations section:


      Private Const SPI_SCREENSAVERRUNNING = 97&
      Private Declare Function SystemParametersInfo Lib "User32" _
          Alias "SystemParametersInfoA" _
         (ByVal uAction As Long, _
          ByVal uParam As Long, _
          lpvParam As Any, _
          ByVal fuWinIni As Long) As Long

      Private Sub Form_Load()
         Command1.Caption = "Disabled"
         Command2.Caption = "Enabled"
      End Sub

      Private Sub Form_Unload(Cancel As Integer)
         'Re-enable CTRL+ALT+DEL and ALT+TAB before the program terminates.
         Command2_Click
      End Sub

      Private Sub Command1_Click()
         Dim lngRet As Long
         Dim blnOld As Boolean
         lngRet = SystemParametersInfo(SPI_SCREENSAVERRUNNING, True, _
         blnOld, _
            0&)
      End Sub

      Private Sub Command2_Click()
        Dim lngRet As Long
        Dim blnOld As Boolean
        lngRet = SystemParametersInfo(SPI_SCREENSAVERRUNNING, False, _
        blnOld, _
           0&)
      End Sub

Press the F5 key to run the program, and click the "Disabled" CommandButton. CTRL+ALT+DEL and ALT+TAB are disabled. Click the "Enabled" CommandButton to enable CTRL+ALT+DEL and ALT+TAB again.

Additional query words: kbVBp400 kbVBp500 kbVBp600 kbVBp kbdsd kbDSupport reboot SetSysModalWindow
KBWIN32SDK KBAPI


Keywords          : 
Version           : 
Platform          : WINDOWS 
Issue type        : kbhowto

Last Reviewed: June 7, 1999