To reset AutoCAD 2012 and older, we used to always direct customers to the following solutions:
You would have to extropolate the registry values to the particular AutoCAD vertical. i.e. MEP, Electrical, Mechanical etc. This process is often called reinitiating the secondary installer. This doesn't work in AutoCAD 2013 anymore and the registry keys are in different locations. Hacking the Registry is so passé these days!
The easiest way to reset an AutoCAD 2013 product is below:
Start > Programs > Autodesk > AutoCAD 2013 - English > Reset Settings to Default
Some helpful reference posts:
Thanks to Shawn Niles for assistance in this tip.