there was a recent thread on alla about bitdefender causing issues. You could try creating an exception for the FFXI folder so it doesn't scan it at all. If that doesn't work, might need to contact them for support. Idk if you will be able to get a refund if you aren't going to use it.
As for the products in use, I used to use a corporate edition of Symantec (got it through a previous employer) until it ran out two years ago. Never had trouble with it...idk if the same would go for their Norton branded products. Some of the AVG suites have caused some wonkiness with the firewall in other programs. I'm sure there are some workarounds, but a few friends never could figure it out.
I've been running Avast (free) for two years with no FFXI issues (I set the exception for FFXI folder). There was a brief hiccup once when it was tripping a false-positive on pol.exe after the initial 6.0 update, but it was quickly resolved after people reported it to them. They have a pretty good support team. The initial 5.x install didn't want to play nice with the PowerDVD that came with my blu-ray player, but we narrowed it down too one particular feature and came up with a way to disable/enable it via a batch file when I wanted to watch movies while they worked on the problem. Problem was fixed with the next version update. It's not the greatest, but it often gets top rankings in the reviews. It's caught a lot of sites that my daughter has gone to and stopped malicious elements from loading...so, pleased with it so far. And it's free for home use (just have to respond to a nag once a year to continue use).
***Note: a lot of free AV software will be just that--Anti-Virus, so you will need another option for firewall. Windows has a built-in one, but some struggle to use it. Zone-Alarm free is simple to setup and gets along with FFXI just fine. Even though it is a very simple firewall, it is good enough for most casual users and very easy to manage--it auto-configures for most popular apps, and will prompt you when new programs are detected reaching to the web, or when the versions of an approved program changes. Other options exist--Comodo, etc. but I have no experience with them. But there are free options out there that can and do get the job done.