[Got from alt.dos.batch newsgroup:]

DOS/Win3x:
ECHO G=FFFF:0000 | debug

Win9x
rundll user.exe,ExitWindowsExec

Win98/Me
rundll32 shell32.dll,SHExitWindowsEx 2

WinNT/2k (with Resource Kit)
shutdown /L /R /T:0 /Y

WinNT/2k (without Resource Kit)
@ECHO OFF & cd/d %temp% & echo [version] > {out}.inf
(set inf=InstallHinfSection DefaultInstall)
echo signature=$chicago$ >> {out}.inf
echo [defaultinstall] >> {out}.inf
rundll32 setupapi,%inf% 1 %temp%\{out}.inf
del {out}.inf

WinXP
shutdown -r -t 0