Fatal errors could be caused by lots of things. Conflicts with other software, corrupt drawings, corrupt installations, graphics issues, etc. Without knowing what you are doing when you get this error, we are just shooting in the dark.
If you suspect your files are corrupt:
http://knowledge.autodesk.com/article/How-to-clean-and-repair-AutoCAD-drawings
To check for graphics issues:
http://knowledge.autodesk.com/article/Verify-for-a-recommended-and-certified-graphic-card-with-a-rec...
To perform a Reset to default:
http://knowledge.autodesk.com/article/How-to-reset-AutoCAD-to-defaults
To perform a Repair on the installation:
http://knowledge.autodesk.com/article/How-to-repair-AutoCAD-Installation
To perform a Clean uninstall and reinstall:
http://knowledge.autodesk.com/article/How-to-remove-all-Autodesk-products-from-a-Windows-system
David Pothier