Have you tried doing a repair from the installation disk? Or check your antivirus software to make sure all AutoCAD extensions are permitted. Usually in these forums, someone has made a change in their system, such as a new antivirus, or running a registry cleaner (other than Windows' own). Good luck!
I hope my info was useful, but if by some odd chance it actually solved your issue, feel free to mark your post as "Accept as Solution". Thanks! 🙂