How to dispose the Win32 APIs in c#.net

Hi,

I am using c#.net to develop a winform application.My winform application is using the below components
1)Win 32 dlls (using System.Runtime.InteropServices)
2)Timers(3 in count) (System.Timers)
3)Excel Interop

The memory of the application is not at all coming down .As timers are running continuosly so i cannot dispose the 
So would like to implement dispose patterns .

Is it necessary to dispose the win32 APIs apart from Excel interop.?
If necessary can you please suggest the best way to call and dispose the win32 APIs.


Some of the Win32 APIs Used in application are listed below.   

        [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
        static extern uint RegisterWindowMessage(string lpString);

        [DllImport("wininet.dll")]
        private extern static bool InternetGetConnectedState(out int netConnection, int val);


        [DllImport("Oleacc.dll")]
        private static extern int AccessibleObjectFromWindow(IntPtr hwnd, uint dwObjectID, byte[] riid, ref Excel.Window ptr);


        [DllImport("WtsApi32.dll")]
        private static extern bool WTSRegisterSessionNotification(IntPtr hWnd, [MarshalAs(UnmanagedType.U4)]int dwFlags);

        [DllImport("WtsApi32.dll")]
        private static extern bool WTSUnRegisterSessionNotification(IntPtr hWnd);

Padma Yeddula