Read Me - they mean it.
Did you read the README file the last time you performed an install? Probably not. The README file for many Autodesk products includes something most people overlook.
If you have multiple AutoCAD-based products on a computer, and you uninstall one or more of the products, you must repair or reinstall the remaining products.
Many people install a new version, leaving the old version on[*] to finish up projects or whatever, and then later decide to uninstall the old version. Chances are something will not be working right in the new version after you perform this UNinstall.
Make sure to run a repair on all remaining AutoCAD based products after doing the UNinstall. To do this, open your control panel, choose Add or Remove Programs, then find the desired product. Click the Change button and follow the prompts.
Of course, this rule isn't limited to "old" and "new" versions of the same product. It would also apply for example in the case of two different products like Civil3D 2006 and Land Desktop 2006.
* Check your EULA regarding how long you can keep two versions installed.