Code:

void __fastcall TForm1::Button1Click(TObject *Sender)
{
    DEVMODE dm;
    dm.dmSize = sizeof(DEVMODE);

    int index = 0;
    while ( EnumDisplaySettings ( NULL, index, &dm ) )
    {
        if ( dm.dmPelsWidth == 800 && dm.dmPelsHeight == 600 )
        {
            dm.dmFields = DM_PELSWIDTH | DM_PELSHEIGHT;
            LONG const result = ChangeDisplaySettings( &dm, CDS_TEST );
            if ( result == DISP_CHANGE_SUCCESSFUL )
            {
                ChangeDisplaySettings( &dm, CDS_UPDATEREGISTRY );
                break;
            }
            else if ( result == DISP_CHANGE_RESTART )
            {
                ShowMessage( "Potreban je restart!" );
                break;
            }
            else
            {
                ShowMessage( "Nepromenjeno." );
                break;
            }
        }
        index++;
    }
}