You probably have found the answer (since it is a rather old post) but just in case, here are the Registry info needed for Oracle 9i:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSDTC\MTxOCI]
"OracleXaLib"="oraclient9.dll"
"OracleSqlLib"="orasql9.dll"
"OracleOciLib"="oci.dll"
Also, don't forget to enable...