Windows Vista - Disable User Access Control (UAC)
User Access Control (UAC) is a highly annoying "feature" in Windows Vista that prompts you to ok almost any change to the system. I think UAC feature like this can causes people to become numb to prompts and automatically click ok on any prompt, which can make the computer more vulnerable to security threats.
Note: Disabling UAC will lead to a less secure system, so be warned. (I have to say this even though I don't believe it.)
To disable User Access Control: