You can do this with a rule. Starting the Rules Wizard will be different depending on your version of Outlook. If you have 2010, you'll go to the Home tab and choose Rules | Create Rule, then go to Advanced Options. In older versions, I think you to Tools | Manage Rules, then click on New Rule. If you go the first route, you'll be in the screen you need to be. If you go the second route, the next step is to choose "Apply rule on messages I receive" under "Start from a blank rule".
In this step you should see "Which condition(s) do you want to check?" You can select the checkboxes for "from people or public group" and "with specified words in the subject". Then in the bottom part of the window click on the link for people or public group. You can select a name from the address book or manually type in the address in the From line, and then press OK. Then click on the link for Specific Words and type Important Notice and click on Add and then OK. Now click next to go to the next set of checkboxes.
In this window, you'll choose what to do when the message is received. You will want to check "have server reply using a specific message." Then click on the "a specific message" link in the bottom. This will open up an email form. Create the message with the subject and body that you want to send out and click on Save & Close. You can also add other actions, like moving the received message to a specified folder, flagging it for follow-up, etc. When you're done in this window click on next.
This step gives you the chance to add exceptions to the rule, like if you don't want specific people to receive the automated reply. Click on Next when you are finished here. Then you can give the rule a name and make sure to choose "Turn on this rule". Click on Finish and you should be good to go. Let me know if this works for you!
