and how did you turn it on?

If you are referring to the Default Application, the one that is used to open a specific file, then changing it is easy. You cannot Turn Off the default application, it is needed by the system to open files.

For instance, when you insert a music CD, iTunes launches and plays the CD. That is because iTunes is the Default Application for playing music.
Likewise iPhoto will launch when you connect your camera, Preview (or Adobe Reader) will launch when you double click on a PDF file.

So, if that is no what you are talking about, try again with a lot more detail. What, exactly, were you doing when you turned it on and what are you doing now that brings up the error message.

Oh, wait a minute, do you mean FILE VAULT? If so, go to the System Preferences, Security and turn it off there.

Any help?

P