One of the reasons for the roll back of SEP in Vista could also be due to Liveupdate version. You can try uninstalling the Liveupdate from add/remove programs. Download the Liveupdate version 3.5 from the following site.
ftp://ftp.symantec.com/public/english_us_canada/liveupdate/3.5
Run the lusetup and it will install Liveupdate 3.5 on your Vista machine and then you can try the installation