Q: Why would I still want to use AIM 5.9?
A: It's really up to you which AIM you choose to use. You might like the look and feel of the 5.9 version better, and depending on your PC, you might not be able to upgrade to 6.0 at this time. Some people just don't like change. AIM 5.9 won't be supported on the Microsoft Windows Vista operating system, while AIM 6.0 will
http://www.aim.com/help_faq/starting_out/getstarted.adp#6_11