Hi,
Unload your existing CUI file. Save it to a safe location. Go to the UserDataCachae folder and copy the ACAD.CUI file to the same location that the orginal CUI file was loading from. Load that as your default CUI file and see if the CUI dialog now comes up. You can find UserDataCache under the Installation Directory of AutoCAD. The CUI file is in the Support folder by default. Don't load it from there but instead copy it to a new location. You have to keep this folder clean. AutoCAD uses it for secondary install.
C:\Program Files\AutoCAD 2006\UserDataCache\Support
Cheers!