AIM has set itself as the default instant messenger for your system.  If
you have the latest update to Windows, you should see a "Set Program
Access and Defaults" shortcut somewhere in your menu structure, if you
didn't delete it.  Click that, or go into Control Panel and select that
option from the list on the left of the CP window.  Change your default
messenger back to whatever you want it to be.

There are also some Registry hacks regarding instant messenger, see:

    http://www.winguides.com/tweak/redirect.php?id=981

for details.