For what it's worth, a similar issue was posted here:
https://www-secure.symantec.com/connect/forums/issues-installing-sep-1106mp2-client-windows2008-x64
It wasn't SP1 for W2K8 64-bit, but the same "CustomAction WriteCcServiceRegistry.D9F570F7_4F5A_41B0_9D99_89851EE85080 returned actual error code 1603 (note this may not be 100% accurate if translation happened inside sandbox)" error was found in your SEP_INST output, although you don't have any 1607 errors. In the previous case, it seemed the deployment package was corrupted, so running a repair on the SEP Manager seemed to work. I can't explain why the package would suddenly become corrupted if you had this installed on several systems previously, and no changes made to the install packages.
Of course, updating to SEP 11 RU7 would likely give the same result (and that version supported by Symantec for that OS, as already proven by other replies), since it would update all of the packages.