If you get an error message when trying to use a Tool palette tool, stating that the tool can not be executed due to the class not being registered, the fix is relatively simple. A repair installation generally clears up the issue and restores your tool palettes. It is a good idea to back up any customized files (CUI, modified out-of-the-box style files, etc), just in case the repair installation overwrites any of those with the out-of-the-box version. Of course, you do that on a regular basis anyway (right?), as well as regularly export your profile(s) in the Options dialog, so that you have current, or nearly current files to restore in the event of corruption or other disaster.
In some cases, it may be possible to avoid a repair installation. Matt Stachoni provides the things to check and the steps to take for this "less drastic" option in this thread in the Autodesk AutoCAD Architecture Discussion Group.
One common way for this situation to occur is to uninstall one version while leaving another version installed.