Add the code below to a module. You can then call ChangeRes to change the resolution, specifying the width and height. For example, ChangeRes 800,600

'Changes resolution on the fly, without rebooting
'Call with:
'Call ChangeRes(800,600)
'or Call ChangeRes(640,480) for example
' if resolution is not possible, a dialog is displayed
Private Declare Function EnumDisplaySettings Lib "user32" Alias "EnumDisplaySettingsA" (ByVal lpszDeviceName As Long, ByVal iModeNum As Long, lpDevMode As Any) As Boolean
Private Declare Function ChangeDisplaySettings Lib "user32" Alias "ChangeDisplaySettingsA" (lpDevMode As Any, ByVal dwflags As Long) As Long

Const CCDEVICENAME = 32
Const CCFORMNAME = 32
Const DM_PELSWIDTH = &H80000
Const DM_PELSHEIGHT = &H100000

Private Type DEVMODE
    dmDeviceName As String * CCDEVICENAME
    dmSpecVersion As Integer
    dmDriverVersion As Integer
    dmSize As Integer
    dmDriverExtra As Integer

    dmFields As Long
    dmOrientation As Integer
    dmPaperSize As Integer
    dmPaperLength As Integer
    dmPaperWidth As Integer
    dmScale As Integer
    dmCopies As Integer
    dmDefaultSource As Integer
    dmPrintQuality As Integer
    dmColor As Integer
    dmDuplex As Integer
    dmYResolution As Integer
    dmTTOption As Integer
    dmCollate As Integer

    dmFormName As String * CCFORMNAME
    dmUnusedPadding As Integer
    dmBitsPerPel As Integer
    dmPelsWidth As Long
    dmPelsHeight As Long
    dmDisplayFlags As Long
    dmDisplayFrequency As Long
End Type
Dim DevM As DEVMODE

Sub ChangeRes(iWidth As Single, iHeight As Single)
    Dim a As Boolean
    Dim i As Integer
    i = 0
    Do
        a = EnumDisplaySettings(0&, i, DevM)
        i = i + 1
    Loop Until (a = False)

    Dim b&
    DevM.dmFields = DM_PELSWIDTH Or DM_PELSHEIGHT

    DevM.dmPelsWidth = iWidth
    DevM.dmPelsHeight = iHeight

    ChangeDisplaySettings DevM, 0
End Sub