Hiya and Welcome
uninstalling the old version may cause a problem. You can backup your database of names.
http://www.icq.com/icq123/backupconlist.html
However, the feeling at the FAQ is just to back it up and install a new version.
Assuming Windows98, as you have not specified, go to RUN and type MSCONFIG. Go to the Startup Tab and look for the older version. Uncheck it, apply and restart. If you are unsure, then let us see your list. Run and type MSINFO32.
On the left look under Software Enviroment, then Startup Programs. Copy/paste the list.
Regards
eddie