Yea I switch off the anti-virus and the game runs fine, the problem isn't the virus anymore, which gets picked up on both ultima, and real versions btw. The actual problem now is random crashes, and no there is no pattern that I can see, it just happens every so often. The last time it happened, I had been playing about 2 hours, was at the gold saucer and was messign around with the bike mini game, and it crashed. If on XP I get FF7.exe has encountered a serious error and needs to close, on Vista I get FF7.exe needs to shut down, this applies also to both legit copy and my Ultima edition.
I think it's just one of those things I'm going to have to live with unfortunately.
Also to answer the question, I use AVG free edition on my laptop (Vista) and Norton 2007 on my desktop pc (XP)