Alrite, this is what you do:
1) Hit New Message, and write in the message that you want sent out to people, don't type in any address, but put a subject, then hit File>Save As> [whatever.eml] (I'll use vacation.eml for this example)
2) Goto the Tools > Message Rules > Mail option.
3) In the first window, scroll to the bottom, and check "For All Messages"
4) In the second window scroll down and check "Reply with Message"
5) In the bottom window, click "message" which is highlighted in blue, and then browse and select the email you saved. Hit OK
6) Type in a name for the Rule, ie: Vacation Auto Responder Hit OK, and then OK again
You're now set, all you have to do is keep Outlook Express Open, and your PC running while you're gone.
Hope this helps you out,
Edit: When you get back, just goto the Tools> Message Rules> Mail and then either Uncheck or Remove the rule you made.
It's the same procedure for Outlook, but wither different menus and such, but generally the same principle with setting up a Rule for all incoming mail.
--Mark