1) Upgrade to the newest Adium release
The newest release of Adium (available at http://www.adiumx.com) may already include a fix for your issue!
2) Remove any non-standard plugins or add-ons
We are unable to test the majority of non-standard plugins and add-ons. Removing these will place your Adium in a more stock configuration and may solve your issue.
Non-standard add-ons are installed in (from your home folder)~/Library/Application Support/Adium 2.0/ . They include Plugins, dock icons, message styles, emoticons, sound sets, etc.
3) Try a fresh set of preferences
Corrupt or old preferences are a good source of crashes and odd behavior. It may also happen that you've managed to configure your Adium in such a way to discover a bug with a specific combination of preferences!
The quickest way to rule out these issues is by temporarily creating a new set of preferences and checking if the problem still persists.
a) Quit Adium
b) Rename the preference folder at (from your home folder)"~/Library/Application Support/Adium 2.0" to "Adium 2.0-old".
c) Relaunch Adium and see if your problem persists.
To restore your old preferences, simply quit Adium, delete the new "Adium 2.0" folder that was created, and rename your "Adium 2.0-old" folder back to "Adium 2.0".
4) Try a fresh user account
Occasionally, an issue in Adium can be caused by a strange setting for your OS X user or other installed software (especially system mods and hacks). Temporarily creating a new user (In System Preferences -> Accounts) is the best way to check against these issues.