Have you even run into a problem in AutoCAD (or a vertical such as Civil 3D), that you can't figure out such as a drawing behaving strange, commands not acting right, commands not working, or maybe even the whole application just won't launch? One of the first things I would suggest is to have someone new log onto your machine and launch AutoCAD under their Windows profile, essentially creating a new "AutoCAD Windows Profile". If the problem goes away when you do this, then it is probably some error or corruption in your own "AutoCAD Windows Profile". Read more about Reset your AutoCAD Windows Profile
At some point in time you may be required to run a repair on your AutoCAD (or other Autodesk product) installation. This is generally required if you UNinstall one version of an application after installing another version. For example, if you install Land Desktop 2006 then later on UNinstall Land Desktop 2005, you must run a repair on your 2006 install.
If you have any unexplained errors that cannot be solved by other methods, such as commands that stop working, a repair may be required also.