Here we start the tutorial:

1. First take ownership of "%windir%\System32\authui.dll" file and then open it in Resource Hacker.

Note: here %windir% represents "Windows" directory which can be found in the system drive where Windows is installed. In most of the cases its C:\Windows.

2. Now press "Ctrl+F" keys or select "Find Text" option from "View" menu.

3. Type Shutdown in Find what text box. Click on "Find Next" button and it'll go to following lines:

Code:
 <element id="atom(ShutDownFrame)" layout="flowlayout()">
    <button id="atom(ShutDown)" accname="resstr(12128)" tooltip="true"/>
    <button id="atom(ShutDownOptions)" accname="resstr(12131)" accdesc="resstr(12131)" tooltip="true" shortcut="115"/>
4. Replace the last 2 lines of code with following:

Code:
 <element id="atom(ShutDownFrame)" layout="flowlayout()">
    <button id="atom(ShutDown)" accname="resstr(12128)" tooltip="true" layoutpos="none"/>
    <button id="atom(ShutDownOptions)" accname="resstr(12131)" accdesc="resstr(12131)" tooltip="true" shortcut="115" layoutpos="none"/>
Guess what? We are only adding an extra string layoutpos="none" at the end of each line.

5. That's it, click on "Compile Script" button and save the file. Now you'll not see the "Shutdown" button on login screen.

PS: Windows Vista "authui.dll" file contains only a single instance of (ShutDown) string, so you'll need to replace it only once using Step 3 and 4 but Windows 7 contains 3 instances of this string, so you'll need to change all the 3 instances in the file.