You didn't say what version of Windows your using. Go to - Start - Control Panel - Windows XP = add or remove icon or Windows 7 = Program and Features - Aol Uninstaller it will search and you can uninstall the AOL you don't want. As how to make one particulliar AOL as your default I don't know. I don't want to search now.
For all old AOL Sign-in software I'll give a download website. I can't say if it will help you.