MSDTC, Active Diretory, DAO, And Transactions

If this is in the wrong forum, my apologies. 

Here is the situation.  We have a .Net application that some
consultants developed several years ago.  They made use of DAO for
database access, using some inbuilt classes like ContextUtil to get a
transation environment.

Until last weekend, the envorinment was:

Web application on IIS5 / Windows 2000 Advanced Server

Database on SQL2000 on Windows 2000 Advanced Server

We switched over to a new database server environment - a cluster of
two Windows 2003 servers wth SQL Server 2000.  This required
Active Directory to be set up on the two database server machines.

Everything worked but this particular application - where it goes to
update the database.  It appears (but since I haven't yet been
able to rebuild the .Net solution) that the error - "Object reference
not set to an instance of object." is in fact a spurious error caused
when the transation tries to roll back.

I have spent about 10 hours on this, and have come to the conclusion
that the fact I have MSDTC involved on a database server that is in an
AD domain, while the web server that is issuing the transaction is
outside that domain is the cause of the problem.  Now, I don't
want to put AD on the IIS 5 web server - we are migrating this to a new
Win2k3/IIS6 server in 5 or 6 weeks time.

Any suggestions?  If I have to, I can move the database for this
particular app back to the non-clustered Windows 2000 server (which is
also outside the AD domain).

Thanks for your indulgance.