Did you do an update lately? maybe that did it or install new software of some type?
Just change the extension .cdr or whatever. see if it sticks?
Shane
I think you hit the nail on the head. Somehow a file with an .AI extension got inadvertently opened with Corel and Windows has changed the association to open .AI file types using Corel.
Easy fix.
Using Windows Explorer (not IE) find ANY old .AI file in ANY old Folder.
Right click the file but DO NOT click on ILLUSTRATOR even though it's likely in the list
INSTEAD select "CHOOSE ANOTHER APP" at the bottom.
Navigate that new app list to find ILLUSTRATOR
Be sure the box "Always use this app to open .AI files" is checked
Click OK
That will open the file in Illustrator and will re-associate the the opening of FUTURE .AI files to open using Illustrator
This is not that uncommon to happen.