When you try to open Task Manager, the following error may occur:

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

Method 1

Click Start, Run and type this command exactly as given below:
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d 0 /f

(better – Copy and paste)

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.

Source: Latest Computer