I would completely empty the plug-in folder. Is it possible that you have an old version of some other plugin in there that is not getting deleted upon uninstall?
Then...I'd look at dropping FileMaker 9 altogether and going with FileMaker 11. It's likely it's more compatible. You can contact me directly if you're looking for licensing assistance...and want the best price. We're FileMaker Business Alliance members.
Helping businesses become more effective, productive, and profitable through custom iPad/iPhone applications, FileMaker app solutions, and web site design.
I remember running version 9 under vista briefly and do not recall any issues during that time. However, version 10 was the first version to state vista compatibility.
I do not believe uninstalling removes the shared location for plugins, I cannot remember the location, have to look it up every time (getting old I guess). I’m sure someone can chime in and confirm this.