As a test, have you tried the instructions in this older article?
http://www.symantec.com/docs/TECH94839
It suggests pointing the TEMP directories at another location (USB drive or network folder) for the install and reverting back afterwards...
Another option is to remove SEP first, reboot, then install the latest version. An upgrade attempts to do a side-by-side install in which case both versions are present on the disk (which obviously uses more disk space).