Remove titlebar from window. Window process started by different user

I am using this code (on windows 2003) to remove and resize window:

Process process = Process.GetProcessById(12121);

IntPtr mwh = process.MainWindowHandle;
SetWindowLong(mwh, GWL_STYLE, WS_VISIBLE);
ShowWindowAsync(mwh, 3);
SetWindowPos(mwh, new IntPtr(0), 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE | SWP_FRAMECHANGED);

And declarations:

[DllImport("user32.dll")]
private static extern bool ShowWindowAsync(IntPtr hWnd, int nCmdShow);

[DllImport("USER32.DLL")]
public static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true, ExactSpelling = true)]
public static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, int uFlags);

static readonly int GWL_STYLE = -16;
static readonly int SWP_NOMOVE = 0x2;
static readonly int SWP_NOSIZE = 0x1;
static readonly int SWP_FRAMECHANGED = 0x20;
static readonly int WS_VISIBLE = 0x10000000;

Everything works correct, when I am resizing window associated with process started by me. But when I want to do this with other users windows, then it does nothing. How to make it works for other users windows?

The behavior is by design in Windows Vista and later, due to the User Interface Privilege Isolation (UIPI). You could solve it if you have access to the source code of the controlled applications.

Read this answer for more: https://stackoverflow.com/a/15445510.