In the group policy editor try:-
Click Start, Run, type gpedit.msc and click OK.
Navigate to...
User Configuration / Administrative Templates / System / Ctrl+Alt+Delete Options / Remove Task Manager.
Double-click the Remove Task Manager option.
Set the policy to Not Configured.
Or in regedit try:-
Click Start, Run and type Regedit.exe
Navigate to...
HKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Policies\ System.
Delete the value named DisableTaskMgr.