A little late, but check for corrupt policies. The agent logs should say that the Symantec Management Agent will no longer start because it's failed three times in a row. If so, back up and delete the local client policy .xml's and attempt to restart the agent. If it starts, the policy was corrupt. Review the backed up client policy and compare it to the new, good policy it just received to see what is different and which policy may have caused the problem.
If the new policy also causes the computer to crash, the problem is still current and you'll want to compare broken client policies to working client policies, assuming at least some clients are functional.