Task Manager has been disabled by your administrator

When you try to open Task Manager, the following error may occur:
Task Manager has been disabled by your administrator

Resolution
This error is caused if the DisableTaskMgr restriction is enabled. To enable Task Manager, try one of these methods:

IMPORTANT: If this restriction was enabled in your system without you doing anything or without your knowledge, then it's highly likely that a Virus has blocked the usage of Task Manager in your system by enabling the DisableTaskMgr policy via the registry. I strongly suggest that you perform a thorough checkup of your system immediately. Steps listed in the Resolution section of this article helps you unblock the Task Manager, but that does not remove the Virus (if any) from your system.

Method 1
Click Start, Run and type this command exactly as given below: (better - Copy and paste)

REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d 0 /f

Method 2
Download and run this REG fix and double-click it.

Method 3
Click Start, Run and type Regedit.exe
Navigate to the following branch:
HKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Policies\ System

In the right-pane, delete the value named DisableTaskMgr
Close Regedit.exe

Method 4:

Using Group Policy Editor - for Windows XP Professional
Click Start, Run, type gpedit.msc and click OK.
Navigate to this branch:
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.