Works fine for me under Win98:

Code:
Private Declare Function SendMessage _
 Lib "user32" Alias "SendMessageA" _
(ByVal hwnd As Long, ByVal wMsg As Long, _
 ByVal wParam As Long, ByVal lParam As Long) As Long
Private Declare Function GetDesktopWindow Lib "user32" () As Long
Const WM_SYSCOMMAND = &H112&
Const SC_SCREENSAVE = &HF140&

Public Sub ActivateScreenSaver()
    
    Dim ret As Long 'junk return value
    Dim desktop As Long 'desktop handle
    desktop = GetDesktopWindow
    ret = SendMessage(desktop, WM_SYSCOMMAND, SC_SCREENSAVE, 0&)
    
End Sub