Go to "System Preferences>User & Groups" and create a new user (you must have admin privileges). Restart under the new user and try running AI. You'll have to move critical files to the new user account by way of the "Public" folder or just by copying them to an external drive.
Hope this allows you to get your job completed.
P.S. Do you still have your install DVD? This would be a more full-proof way to install the software. You'd have to run the updates though.