Cause: There are different reasons for it please look below for more info
There are different reasons why you could be seeing this issue:
1. Because someone changes the group policy.
To fix this issue run the following command from the RUN box:
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableRegistryTools /t REG_DWORD /d 0 /f
use group policy editor: gpedit.msc from run box and then look for policy called "Disable registry editing tools " set it to not configured state and restart computer
2. This could also be caused due to different viruses/malware
to fix this there is a nice script from Symantec just use that
even I could export my shell commands configuration in registry and send you as a TXT file but I think Symactec tool already does the trick for you.