Auto Reply or "Out of Office" notices can be sent using a message rule. Tools>Message Rules>Mail>New.
Check "For all messages", action "Reply with Message". You must then create a *plain text* message (my_message.txt) to respond with. Using auto-responses of this nature has some serious drawbacks.

1) You have just notified anyone who Emails you that your home is vacant and how long they have to empty it out.
2) The response will also be sent to Spam received with the following results.
    a) The sender gets verification that your address is valid so you go on the hot list. This seldom happens because most spam return addresses are fake.
    b) Because most spam reply address are fake your message "bounces". You receive a message from the postmaster that the mail could not be delivered, and your auto-response answers that and it bounces back again ad infinitum. The end result is a plugged inbox.

A slightly better rule is to replace "For all messages" with "Where the from line contains people". Click on "people" in the bottom box and click "Address Book". Select all of the names in your address book and add them to the list. This is assuming that you have kept your address book up to date. If it contains invalid addresses then see "b)" above. Both methods require that your computer be left turned on with Outlook Express running 24/7 ...*and* connected to the 'net *and* set to poll for new messages on a schedule