Couple things suggest that it's a permission issue, Like you are able to install properly and the service starts up fine, 'cause this is done under the LSA account even if you launch it interactively.
1) Check the "Allow service to interact with desktop". Reboot. If it doesn't help, try 2)
2) Get the LSA, This is how. Type
at 11:11 /interactive cmd.exe
on a command prompt
11:11 is just the time, you can type anything.
Goto c:\windows\tasks and run the task. A command prompt under LSA will be launched(svchost.exe)
Type
taskkill /im explorer.exe /f
in the svchost.exe cmd window and then
explorer.exe
in the same window.
You are logged in LSA now. Any better?
3) If 2) resolves it, request for testsec.exe from Symantec, or if you have any of their ESUG application(Like cleanwipe, sylinkreplacer), unpack it from there by renaming the exe to zip. Execute it and see where all there are permission problems.
http://service1.symantec.com/SUPPORT/ent-security.nsf/ppfdocs/2008053006154648?Open&dtype=corp&src=&seg=&om=1&om_out=prod
I am not 100% sure that 2) would give a definite idea as it's a logon and not reboot, but in any case run the testsec.