If you want to get rid of the annoying "Aim Today" window that pops up every time you sign on AIM, first bring up your AIM Preferences window by clicking the preferences icon in the Buddy List toolbar

or by right clicking your system tray AIM icon.

Now select the "Sign On/Off" category on the left hand side, and then uncheck the box that says "Show Today window at signon".

Close the preferences window and the next time you restart AIM, the "AIM Today" window will not pop up!