Very Honestly speaking Now since you have upgraded from Outlook 2003 to Outlook 2007, but you forgot the server side. There are many broken functionalities when outlook 2007 connects to Exchange 2000.
Even Microsoft is ending server 2000 and exchange 2000 support by July 13.
I suppose the best would be to upgrde the exchange server which will sort out all the issues.
Because you will face the DST issues as well when the DST period ends. Not only this but even the Free/ Busy issue will alays come.
My suggestion would be to go for Exchnage 2010 if you are looking for upgrade.