RegClean analyzes Windows Registry keys that are stored in a common location in the Windows Registry. It finds keys that contain erroneous values, and after recording those entries in an UNDO.REG file, it removes them from the Windows Registry.
What RegClean Does:

RegClean analyzes Windows Registry keys that are stored in HKEY_CLASSES_ROOT in the Windows Registry. It finds keys that contain erroneous values, and after recording those entries in an Undo.reg file, it removes them from the Windows Registry.

What RegClean Does Not Do:

RegClean does not fix every known problem with the registry. It does not fix a corrupt registry; it is limited to fixing problems with normal Windows Registry entries located in HKEY_CLASSES_ROOT.

It is very possible that RegClean will not correct a problem that you have encountered. RegClean will leave any entries in the registry that it does not understand or that could possibly be correct.