Outlook Express does not have the out of office (OOF) facility that it's big brother has. However, you can create a rule that it emails the sender a message. You will just have to remmber to switch it on and off. First create the message and save it as something eg OOF.eml then in tools > message rules > mail > tick "for all messages" > tick "reply with message" then select the OOF.eml message. Save it and you are away. Just remember to de-select it when you are back at your desk.
It should appear on the Tools section of Outlook express - if it does not check out this website for details of what to do - havent tried it myself, but seems fairly self explanatory! http://support.microsoft.com/kb/311107