Welcome to the site!
1) DO A SYSTEM RESTORE TO BEFORE YOU INSTALLED 5.5 (This will remove 5.5's entries from your registry)
2) DELETE ALL FOLDERS FOR AIM YOU CAN FIND (Shortcuts,etc)
3) Run REGSEEKER to clean up any remains (Might have to set it to XP compatibility im not sure) (BEST REG CLEANER OUT THERE)
Good luck.........