Microsoft Office (XP and 2003) have built-in voice recognition that allows you to dictate text.
You could look into something like Dragon NaturallySpeaking for one that allows both dictation and voice commands.
If I remember correctly, Dragon NaturallySpeaking supports voice commands to open up applications etc. So you should be able to say something like "start notepad" and it opens up Notepad.
Of course, "start computer" wouldn't work, since Windows needs to be running to actually receive the voice commands.
get a ma.. oh no, ehm, What geffy said really. Dragon Naturally Speaking will handle voice commands. It's a good piece of software, but quite expensive if I remember (and there are no alternatives for free, that I know of)