Uninstalling Programs

OK I am having a major issue here....how in the world do I uninstall programs....I downloaded a program called DIVX and it was working for what I downloaded it for but it is nowhere to be found on my harddrive to uninstall it and it is not in my applications list. does anyone know where it might be on my hard drive? but aside from that alot of programs that are in my applications list do not even have an option to uninstall. I use firefox but if I ever wanted to uninstall it there is no uninstall option, same goes for programs like AIM or Yahoo Messenger.

 

I am fairly certain I originally downloaded it from the Divx site. I was linked from a site Icefilms.info. but it was awhile ago. Like I said I downloaded it awhile back and it has been working fine, but then there was an upgrad that did not work in this site and I want to uninstall it and reinstall the older version. there is no Divx in my applications list, none. perhaps when I downloaded it I did not drop it into my applications, but would it even work if I didnt do that? regardless how can I get rid of it if I cant find it? I did find in my internet plug ins folder, in my library folder it says npdivx.xpt....an executable file...any idea what they might be? when I go to open it it tells me to choose a program to run it with.