Outlook 2003 with Exchange 2000 issue

I am running a recently part migrated network 2K3 & 2000 servers, exchange 2000 servers win2kpro and xp clients running office 2k & 2k3. we have an issue on the xp clients that everytime a user logs in to an xp box for the very first time, then user trys to open outlook, it hangs at creating the welcome message, but will eventually open ok, does anyone know what causes this issue?