Changing the screen resolution in Windows Vista

Hello all. I hope someone can help me. I am working on an application for Windows XP and Windows Vista, and I need to change the resolution of the screen. The code I am using works fine in XP, however it fails in Vista. If someone could please help me I'd appreciate it. Thank you very much.

Below please find my code sample:

public CResolution(int a, int b)
        {
            Screen screen = Screen.PrimaryScreen;


            int iWidth = a;
            int iHeight = b;


            DEVMODE1 dm = new DEVMODE1();
            dm.dmDeviceName = new String(new char[32]);
            dm.dmFormName = new String(new char[32]);
            dm.dmSize = (short)Marshal.SizeOf(dm);

            if (0 != User_32.EnumDisplaySettings(null, User_32.ENUM_CURRENT_SETTINGS, ref dm))
            {

                dm.dmPelsWidth = iWidth;
                dm.dmPelsHeight = iHeight;

                int iRet = User_32.ChangeDisplaySettings(ref dm, User_32.CDS_TEST);

                if (iRet == User_32.DISP_CHANGE_FAILED)
                {
                    MessageBox.Show("Unable to process your request");
                    MessageBox.Show("Description: Unable To Process Your Request. Sorry For This Inconvenience.", "Information", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
                else
                {
                    iRet = User_32.ChangeDisplaySettings(ref dm, User_32.CDS_UPDATEREGISTRY);

                    switch (iRet)
                    {
                        case User_32.DISP_CHANGE_SUCCESSFUL:
                            {
                                break;

                            }
                        case User_32.DISP_CHANGE_RESTART:
                            {

                                MessageBox.Show("Description: You Need To Reboot For The Change To Happen.\n If You Feel Any Problem After Rebooting Your Machine\nThen Try To Change Resolution In Safe Mode.", "Information", MessageBoxButtons.OK, MessageBoxIcon.Information);
                                break;
                            }
                        default:
                            {

                                MessageBox.Show("Description: Failed To Change The Resolution.", "Information", MessageBoxButtons.OK, MessageBoxIcon.Information);
                                break;
                            }
                    }
                }

            }
        }