SHAMELESS COPY AND PASTE FROM VIRTUALPLASTIC.NET http://www.virtualplastic.net/html/misc_cut.html (THOUGHT THIS WAS IMPORTANT AND SHOULD BE HERE TOO):

Enter this command line:
rundll32 user.exe,exitwindows
..to exit Windows easily,

rundll32 user.exe,ExitWindowsExec
..to restart Windows,

rundll32 shell32.dll,SHExitWindowsEx 2
..to reboot the computer, and..

rundll32 shell32.dll,SHExitWindowsEx
..to log off the current user (without the confirmation question).
Those are for 9x (95, 98, Me). XP has shutdown.exe, also available in NT and 2k resource kits:
%windir%\system32\shutdown.exe -r
would be a reboot. Some useful attributes:
-r = shuts down and restarts;
-f = forces processes to terminate;
-l = shuts down all running processes, then logs off;
-s = shuts down to a point at which it is safe to turn off power;
-h = help.

Also note the "-t xx" attribute (thanks Clockwise). Default shutdown has a countdown of 30 seconds, with this you can change that ("-t 00" being zero).

Hope it helps you Mr. White, and DeViLbOi, if you think i should leave only the link to virtualplastic tell me and i'll remove the rest.