Are you using Windows Mail? This is for Outlook Express, but it is the same procedure.
If you do this, you will have to leave your computer on 24/7, be connected to the Internet, and have your e-mail program open. If you want do this anyway, the link below will explain how to do it. One thing I would suggest is that where it says to create a rule and check For All Messages, you change it and make the rule for people in your address book only. Otherwise you will be replying to Spam as well and could end up in an endless loop of bounces.
Most ISPs/Mail Servers offer this service which would be a better choice.
How to Set Up a Vacation Auto-Response in Windows Mail, Outlook Express, or Windows Live Mailhttp://email.about.com/od/windowsmailtips/qt/Vacation_Auto_Response_in_Windows_Mail_or_Outlook_Express.htm
Configuring Outlook & Outlook Express To Auto Respond to Incoming E-mailhttp://www.onecomputerguy.com/app_info/outlook_autoresponse.htm
Bruce Hagen
MS-MVP Oct. 1, 2004 ~ Sept. 30, 2010
Imperial Beach, CA