Fix a Windows Task Manager that was accidentally disabled
1. Click Start | Run. 2. Enter gpedit.msc at the command line and click OK. This will open the Group Policy settings window. 3. Select User Configuration | Administrative Templates | System | Ctrl+Alt+Delete Options | Remove Task Manager. 4. Double-click the Remove Task Manager option from the Group Policy menu. You can then disable, enable, or set the policy to Not Configured. Remember: Since the policy in question is called Remove Task Manager, by disabling the policy, you are actually enabling the Task Manager. Disabling or setting this policy to Not Configured should alleviate your problem.
Another way to repair this problem
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
