Force a shutdown in Windows

rundll32.exe shell32.dll,SHExitWindowsEx 4

The above will shutdown any running processes and shutdown your computer without saving any data or warning. It should only be used if you have problems shutting down normally.

Note the numbers at the end of each of the above command lines. They are what make the different actions possible and they can be combined. The most common combination is using the Force shutdown action with a restart. Restart = 2 Force =4, so you would just add 6 to the end of the main command line:
rundll32.exe shell32.dll,SHExitWindowsEx 6

If your computer supports the power-off feature, you can also use the number 8 which is a Power Off action which will not only shutdown your computer, but turn off the power too.rundll32.exe shell32.dll,SHExitWindowsEx 8

To summarize:

0 = Logoff
1 = Shutdown
2 = Restart
4 = Forced Shutdown
8 = Power Off (Shutdown and turn off Power)