If you are using COM+/Enterprise Services components that talk to resource managers like Oracle or other XA RM, on Windows Server 2003 you need to make some additional steps to allow this scenario to work. This is due to a security change in MSDTC that requires you to register the XA DLL in registry. Please see more details at http://support.microsoft.com/?id=817066.
Also, make sure XA transactions are enabled. Use "Administrative Tools\Component Services\My Computer\MSDTC\Security Configuration" UI to verify.
Since MSDTC must run now under NT AUTHORITY\NetworkService account, you need to add the "machine account" of the machine where MSDTC is running to the list of users that can do recovery on the XA database in order to allow MSDTC to perform recovery (see http://weblogs.asp.net/florinlazar/archive/2004/01/02/47239.aspx for more details). In the same topic, since NetworkService is a very restricted account, you also need to provide the NetworkService account with access to the folder where the XA DLL is located (see http://support.microsoft.com/default.aspx?scid=kb;en-us;816633).
[Updated Jan 9, 2004][Originally posted Oct 16, 2003]