If you don't connect to an Exchange server, there is no automatic out of
office autoreply. You'd have to spoof it with rules, and note that this
means leaving Outlook open, and connected to the internet, for the duration
of your absence.
Also see http://support.microsoft.com/?kbid=196718 - it explains how to set
up a template so that autoreply fires only once per sender (as Out of Office
does) to avoid mail loops.
The Out of Office Assistant is available in Outlook only when you connect to
an Exchange server. Otherwise, you have to create a rule that emulates it
and leave your Outlook running at all times while you're away. Go to http://support.microsoft.com/, select "Outlook" as the product and enter
"emulate assistant" as the search string. Read the results.