How to enable ' Task manager' in win xp, win vista, win 7 :-

Click on "Start" button, and type "regedit" in run.

Browse the list as 

HKEY_CURRENT_USER\Software\Microsoft\Windows\Current Version\Policies\System.

if the registry named "disabletaskmgr" already present then press 'delete' button on your keyboard.

If not then:-

Create a new 32-bit DWORD value.

Name it DisableTaskMgr.

Double click on it to set its value data to 1.

Enable Task Manager in Windows 7

Double click on the value named DisableTaskMgr to set its value data to 0.

How to enable regdit:-

 Click on "Start" button, and type "gpedit.msc" in run.

click on user configuration

administrative templates

System

scroll down and right click on ''prevent access to registry editing tools''

EDIT and then DISABLED.

then right click on "prevent access to the command prompt"

EDIT and then DISABLED.

How to enable regedit, cmd, gpedit :-

Download this .rar file.

extract it anywhere you want.

Run the file regtools.vbs inside it.

if the message comes 

"registry editing tools are now disabled"

Re-open the file.

If the message come

"registry editing tools are now enabled" then

Click on "Start" button, and type "regedit" in run.

Browse the list as 

HKEY_CURRENT_USER\Software\Microsoft\Windows\Current Version\Policies\System.

if the registry named "disableregistryeditingtool" something like this only present then press 'delete' button on your keyboard.