If you do this, don't do a repair install. Do a fresh install of Vista 64. It will take more time, but it will be better in the long run. A repair install will borked any newer third-party software ...

I've been running Symantec Corp AV ver 10.2 on 2 vista machines at home for awhile now, they've been fine. A few weeks ago windows security alerts started saying it can't find an AV program.