Windows XP users have turned on Automatic Windows Update will have absolutely no problem recognizing the "Restart Now, Restart Later" Dialog shown in the screenshot below:

Do you want to restart your computer now - Windows Update Restart Now

When Windows XP is finished downloading and installing new updates/patches from Microsoft Windows Update website, it will ask you to restart the computer to complete the installation.

The Restart Dialog Window can be removed by killing the wuauclt.exe process from Windows Task Manager or SysInternals Process Explorer but the most irritating part is that the window pops-up again after few minutes.

There are two solutions to get rid of this annoying Restart Window depending on what version of Windows XP you are using.

The easiest way is to start Windows Command Prompt (Start -> Run -> cmd) and type the following command:
sc stop wuauserv
The Restart Window will disappear immediately including the icon in the system tray.