Note - Do not use Word as your email editor and select plain text for the email format.  These settings are under Tools -> Options, then click on the Mail Format tab.

1)  Make a Template

Open a new blank email and write your message, including the subject and body of the response.  For example:

Subject:  "Out of the Office Until November 15th"
Body:  "Hi. I'm out of the office until November 15th.  I will respond to you as soon as I get back in.

Thanks
Tom"

2)  Once you've completed the message, click on File -> Saves As.  Save as type Outlook Template (*.oft).  It will change the location to the Templates folder - do not change this.  Give the template a name and click the Save button.  Close the email, and say No when it asks if you want to save it.

3)  Next, use Rules Wizard to make the rule.  Go to Tools -> Rules Wizard.  Click on New.  In the first step, it asks what conditions you want to apply to this rulle - leave them all blank and click Next.  Outlook will then tell you that it will apply to all mail received - click yes.

To activate and deactivate the rule, simply go to Tools -> Rules and Alerts, and check the rule to make it active, and uncheck to disable it.