ok thats fine. I personally would not trust AOL at this point to be around in 2-3 years.
Did you try look at the icon link you say you normally use to launch your email? (right click on the shortcut, properties, find target) What did it say? Was the program in the destination location? Windows is telling you it can not find/access the program. You need to look in the location the shortcut is saying and see if it is there. If is not there you may need to reinstall the program. If it is not there you will not be able to register it as the default program like you were trying to do. You said you upgraded from one version to another it sounds like the upgrade didnt work correctly?