Password Prompts Randomly RPC over HTTP

Hello Experts,
We are running Exchange 2003 with RPC over HTTP setup. Clients running
XP/Outllook 2003 have no problems but recently got a new Vista Home
Basic/Outlook 2003 that is giving problems. When we open Outlook, it prompt
us for username/password and we get connected. Then a few minutes later it
prompts for password again. It happens randomly and often. Other XP clients
on same network do not have this problem so it is not a connection problem.
Any help would be greatly appreciated!

Thank you!