When all your appointments are gone in Outlook 2007 or Outlook 2003, this is probably the result of AutoArchive settings enabled. When you have turned on Auto Archiving, Outlook automatically archives all your calendar items, older than x months. Typically, Outlook saves your calendar items to archive.pst.
When you have verified that your Auto Archive settings in your Outlook client are disabled, it may be that you synchronize your email and calendar with your pda and another machine. In that case, make sure that AutoArchiving is also turned off on the other client.