First of all depending on the group policies set on your domain (if your on one) you may not have permissions to set out of office so that's the first thing to check, if your using the out of office assistant and still having trouble turning it off or on give the following tips a try.
1. Outlook Switches - Clean rules ( backup your rules first)
the most common reason for out of office messages not working are that htere a problems with outlook rules either blocking or preventing the out of office assistant from doing its job, so first of all run
outlook /cleanrules
or
outlook /cleanclientrules
if this fails the most likely reason is that you didn't install outlook to the default directory, if that is the case simply use the full path to Microsoft outlook, e.g C:/microsoft/outlook.exe /cleanrules.2. Set your office out office via web mail interface
Next step is to try setting up the out of office assistant from the microsoft exchange web interface just to help pin point the cause of all the troubles.
3. The other possible reasons
The next most common reason I see out office failing for is due to power outages or system crashes. especially relating to ISS services which can be interrupted during incorrect shutdown or crash and a restart of your mail server can usually be an easy fix.
if you still having troubles there are many ways of turning on or off the out of office assistant, try the following Microsoft applications which can help manually edit MAPI settings
http://www.microsoft.com/downloads/details.aspx?FamilyID=55fdffd7-1878-4637-9808-1e21abb3ae37&DisplayLang=en
http://www.microsoft.com/downloads/details.aspx?FamilyID=3d1c7482-4c6e-4ec5-983e-127100d71376&DisplayLang=en
