I was having some issues with some of my VST's in 8.01 and it turns out they had decided they were no longer registered. Loading them up in Live 7 again and all was well. So I tried to re-register them while in Live 8 (in case they thought they were running under a different user account which I've seen do similar things) and it gave me permission denied errors on one of them trying to write a particular file.
Luckily turning UAC off, loading up live8, registering the VST's and then turning UAC back on seems to have worked. However I am wondering why this is an issue at all since there's no such problem with Live 7?