SetWindowLong failed with 'Access is denied' when invoked from a windows service

[dmihailescu]

the P/Invoke SetWindowLong failed with 'Access is denied' when invoked from a windows service.
The service runs under the System account with the 'allow user to interact with the desktop' checked.
If I have the same code executed from a regular user app it works just fine.
Other P/Invoke calls from the windows service are just fine under the system account like: IsIconic, GetWindowRect, PrintWindow, GetClientRect, CreateCompatibleDC, CreateCompatibleBitmap, SelectObject, etc.
So far only SetWindowLong fails in the code below when under system acccount in the service:

 
[DllImport("user32", SetLastError = true)]
internal static extern int SetWindowLong(IntPtr hWnd, int index, int dwNewLong);
//......hWnd is some window on the local desktop
int res = SetWindowLong(hWnd, GWL_EXSTYLE, winLong | WS_EX_LAYERED);
if (res != winLong)
                {
                    Win32Exception ex = new Win32Exception(Marshal.GetLastWin32Error());
                    throw new ApplicationException("SetWindowLong failed: " + ex.Message, ex);//fails in win service, not regular app
                }

Trying to impersonate the logged on user did not work either.
Any ideeas how to get arround?