Verification of prerequisites for Active Directory preparation failed

Question

We currently have Windows Server 2003 SBS, SP2, Domain Controller. Would like to add Windows Server 2012, Standard, 64-bit as a backup domain controller.
"Verification of prerequisites for Active Directory preparation failed. Unable to perform Exchange schema conflict check for domain sxxxx.local.
Exception: The RPC server is unavailable.
Adprep could not retrieve data from the server name.xxxxx.local through Windows Managment Instrumentation (WMI).
[User Action]
Check the log file ADPrep.log in the C:\Windows\debug\adprep\logs\20130417103902-test directory for possible cause of failure."
What the log says is really:
"Adprep encountered a Win32 error. Error code: 0x6ba Error messa The RPC server is unavailable."
Can anyone has similar experience shred some lights to troubleshoot this? Have reviewed
other links that have similar probems but that doesn't help. 
Many Thanks!

Answer 1

Of course I CANNOT remove Symnatec as Meinolf suggests. That would be out of my mind!! I tried to stop all their services though which doesn't help. I know this has nothing to do with Symantec. Here comes another test, the final one:
Test 8
This article is really good as it concludes very thoroughly about the problems about "800706BA - RPC Server Is Unavailable" and other WMI query issues:
http://goo dot gl/l2iha
I started looking at he ISA 2004 on our SBS 2003.
Tried to disable the RPF Filter:
a. Open Microsoft Internet Security and Acceleration Server 2004
b. Go to Configuration > Add-in and location RPC Filter on the right side, right-click on it and select Properties, uncheck 'Enable this filter'
c. Hit Apply....
d. Now I go back to Windows 7 and test the WMI query.
The result: it WORKS! 
e. Next, I tried that on the Windows Server 2012 like so:
c:>wmic /node:sbs2003servername computersystem list brief /format:list
It also works!
f. Next also on Windows Server 2012, I continued on what was left over.  I did the "Rerun prerequisites check " and no surprise - "All prerequisite checks passed successfully. Click 'Install' to begin installation"!
Well that concludes the problem of installing Windows Server 2012 (standard) as a backup domain controller to a Windows SBS 2003 domain controller and the  troubleshooting process that finally led to a solution that solves my problem. Thanks for all
the discussions over the web. Every bit counts!
Well if this helps you in some way, give me some points to buy beer! I am going to have a drink with Bill, Cheers!