We have a user that is currently unable to connect to a SQL Server database through an ODBC connection.  He was, however, able to connect last week.  He has been able to connect for the past 2 months.  Nothing has been changed manually either by the user or admins.  He is allowed access to the database as a member of an Active Directory Group (DB_Users) and all other members of this group have absolutely no trouble connecting to the database.  Whenever he tries to access the database, he gets the following error:

[Microsoft][ODBC SQL Server Driver][SQL Server]Login failed for user '(null)'. Reason: Not associated with a trusted SQL Server connection.

Like I said, he's been fine with the same settings for the past two months, but all of a sudden the user gets that error when trying to connect.  This is probably the 3rd or 4th time we've had this problem and we end up deleting his profile from the computer and start from scratch.  Logging into the pc as a domain admin provides a successful connection which leads me to believe that the issue is somehow related to the user's setup, but I've pretty much run out of things to check.  Any suggestions to a possible solution would be greatly appreciated.