This has been posted many times, Plz search before you post. Anyway here is what you have to do.
It may be due to a Trojan called "RONTOKBRO" or someone might have actually disabled it.... If its the first case scan your computer with a good antivirus ,note that you have to update your antivrus database daily.... If its the second case, to re enable regedit ...
Type gpedit.msc in RUN
go to
User Cofiguration->Administrative Templates->System
Now in the right hand panel set "Prevent access to Registry editing tools" to not configured or disabled.......This will enable registry editor ...Once this is done
Now start registry editor and Navigate to
HKEY_CURRENT_USER\Software\Microsoft\Windows\Current Version\Policies\System
In the right hand panel if a DWORD called "DisableTaskMgr" is present set its value to zero......