This workaround seems to be working most of the time:
The Aug. 2012 workaround:
Using a privileged logon, open the SEP interface. Just before the installation, click the bottom right “Options” button on SEP. In the flyout, click “Disable xxxxxx”
Move upward through each “Option” button you see, clicking “Disable xxxxxx”
If you wish to confirm, click each “Option” button again and observe the phrase ““Enable xxxxxx” instead of “Disable xxxxxx” Proceed with your software installation.
One catch with the workaround is that sometimes at Network Threat Protecction, the Options button has a grayed-out choice for "Disable...."
In those cases greg12 's solution at this thread https://www-secure.symantec.com/connect/forums/sep1211mp1-disable-symantec-endpoint-protection-failed#comment-7498371 seems to be working ... ..
... and that is to "enable or disable the firewall under Status > NTP > Options > Change settings > Firewall."
One other very strange thing about those Options buttons mentioned above, is that sometimes when you click one the wording is of the form "Disable all Network Threat Protection features," and sometime it is of the form " Disable NetworkThreat Protection."
Thanks