Outlook 2007 changes login credentials automatically.

I have run into a problem that I have no idea how to troubleshoot. We have configured an Outlook 2007 client on a workstation that is located in one of our retail stores. Every 3 weeks or so I get a call from them saying that they are not able to receive email.

They receive an error from the incoming mail server.

When I look at the settings two fields have been changed.

1) The incoming name server is changed from "mail.usinternet.com" to "localhost".

2) The username is changed from "corren" to "corren/mail.usinternet.com".

When I make the change to set the incoming mail server back to mail.usinternet.com and the username to corren it works.

Every time they have problems, this is the change I notice that has been made. I am 98.99999% certain that it is not a case of an employee changing the settings.

How does this happen? It seems to happen on it's own.