I am having exactly the same problem and have not seen too much on this
topic. I am using Exchange 2003 (Just upgraded to SP2) and all
clients are Outlook 2003 running on XP. Also the XP machine having the
problem was recently upgraded from 2000 to XP Pro. The messages are
all internal but cannot be accepted. The buttons on the invite are all
there, but they seem to have no affect when clicked. This occurs if I
click Accept, Tent. or Decline. Also not working in the Actions menu
either??