Outlook 2003 keeps unsorting emails

Ok I have a strange problem

I have the exchange 2003 server running in 2003 AD environment.  Few weeks ago one of my IT colleagues at work told me that every so often about every other week, his messages in outlook get unsorted.  In more detail, his emails come into his inbox, he has many folders that he uses for filing message after they arrive into his inbox, so email comes in and after he reads them he files them to individual folders which he has created.  A week or so later he logs into outlook and notices that all the emails that he filed away for the previous week or two are back in his inbox??  Ever single email.  ( this also goes for any emails that he filed away from his sent items)

This has been happening for a couple months now he tells me.  He is using a dell laptop, XP SP2, with Office 2003 all the latest updates including the SP2.  XP also has all the latest updates as well.  There is nothing wrong with his machine, no errors or anything visible that is showing up.  I thought that something was wrong with his Outlook client at first but then one day he went on a business trip and was using the Webmail outlook client and after he logged in same thing happened to him.

I have never heard of such a problem until now, has anyone seen this strange behavior with outlook before??  I guess the first step will be to repair office but before I do any repair/installs of office I thought to ask. Also his mailbox is not big at all, it’s under 1GB in size.

Thanks,