You cannot set the COM RunAs password on a Windows 2003 machine from a Windows Server 2008 R2 machine

The MSDN doc on LSA_UNICODE_STRINGstates the length should not include the terminating NULL character. However COM incorrectly includes the NULL terminator in the Length of the KeyName of the LSA_UNICODE_STRING structure when it calls the LsaRetrievePrivateData function to retrieve the password. In order for COM to find the matching secret name, the DCOMPerm sample includes the NULL terminator for the KeyName that is passed to the LsaStorePrivateData function. This approach works on the Windows 2003 environment but breaks from Windows 7/Windows Server 2008 R2 to Windows 2003. The cause is a change that was made on LsaStorePrivateData and LsaRetrievePrivateData on Windows 7/Windows Server 2008 R2 for robustness improvement. 

The change strips the trailing NULL from LSA secret names. The trailing NULL character is removed if you attempt to create a secret from Windows 7/Windows Server 2008 R2 with the trailing NULL character. The COM code on Windows Server 2003 is searching for the secret with the trailing NULL when retrieving the password. It either finds the secret with the trailing NULL in the registry but the password stored in the secret is not set by your call and is not for the current COM server identity or it can’t find the secret with the trailing NULL at all and thus LsaRetreivePrivateData fails with error code 2 or file not found. In either case, the COM server will fail to run.