ISNULL on NTEXT causes fatal exception on SQL Server
I think you might be a victim to problem I have seen before, introduced with Cumulative update for Service Pack 2. Is the compatibility level of your database 80 (SQL 2000)?

See Microsoft KB 938102 for Hotfix details, but I suggest you upgrade to the latest service pack (or definitely a later cumulative service pack).

Essentially the Query Engine is evaluating your where clause and creating joins using non-ANSI operators such as *=, you then fall foul of this Sql bug. The problem occurs if the following conditions are true:


The query references a view that uses non-ANSI outer join operators,
For example, the view uses the = operator or the = operator.
The column list of the SELECT statement in the view contains a
subquery.
The version of SQL Server 2005 is SQL Server 2005 SP2 or a later
version.
You set the database of SQL Server 2005 to use compatibility level
80.