We have recently upgraded our domain and forest to server 2008 R2, all DC's run 2008 R2 SP1, 6 exchange servers 2003 and outlook 2003 clients.
Several users around the world and various intervals that outlook failed to connect to the exchange server, a window appears requesting network credentials and if they cancel that window then outlook goes offline - Obviously this is quite annoying.
Exchange has been centralised to the datacentre, with cached exchange enabled for all users.
As part of the domain upgrade we removed 10 WINS servers from the local sites and centralised WINS at the data centre, with only now 2 WINS servers.
MS say dont install anymore WINS servers, we have enough but after reading some articals exchange and outlook 2003 require WINS.