DB2 Distributed transaction

I have a Sql2005 server with HIS 2006 installed on (server1). I define a linked server (using odbc or oledb) to a DB2 database (on server2). I have already configured msdtc network access on server 1. I can execute select, insert or update statement from query analizer (with the openquery command) but i can't execute the same statement from a trigger: the operation could not be performed because OLE DB provider XXXX for linked server XXXX was unable to begin a distributed transaction.
Any help would be VERY appreciated.
Thanks