You could use Turn Windows features on or off to turn off .net 3.5 then reboot the machine and turn it back on again, this should fix any .net 3.5 related issues with Autodesk 2011 products.
This could help fix following AutoCAD errors
Unable to run AutoCAD. This may be due to one of the following reasons
1.This version of AutoCAD was not installed properly
2.You do not have sufficient rights to run AutoCAD