Taskkill is Microsoft's own task-killing command line tool. Very useful for creating batch scripts to shut down multiple processes at once, but unfortunately it's only available for Windows XP Pro, not XP Home (don't know about Vista).
Luckily, the blog Buckinwell Fritish has a link to download Taskkill for those that don't have it! Just install and put in the Windows directory, and you're all set. The simplest command to kill a process is this (you can run it from the command prompt, or from doing Windows>Run):
taskkill /F /IM notepad.exe
notepad.exe can be any task you want to kill, like thunderbird.exe, firefox.exe, etc. For more documentation on it, check out Microsoft's official word.
Taskkill does basically the same exact thing as previously mentioned PSKill, but this just seems simpler and more organic than PSKill. When I have the time I plan on converting all my other scripts to use Taskkill and probably deleting all the PSTools as I don't have much of a use for the rest of them.