I haven't come across this solution anywhere so I thought I would post it here. Initially I would have a problem where I would lag horribly. Animations wouldn't load, music would loop in 5 second intervals, menus like the AH wouldn't load, etc. I tried everything including a reinstall, new video drivers, sound card etc and nothing worked.
Well recently I downloaded process guard for that whole "can they detect windower" issue with the new PoL. Everytime I started to lag my processor guard was detecting a process. I don't remember the name exactly but I looked it up and found out it was a process that indexes your hard drives for fast searching. You can disable it in your hard drive properties. Ever since I disabled it my FFXI has worked perfectly.
Here is a SS and I hope this helps people.
http://www.pbase.com/keltiks/image/7...0/original.jpg
XI Wiki

