Enable Task Manager:

Click Start

Click Run

Enter gpedit.msc in the Open box and click OK

In the Group Policy settings window

Select User Configuration

Select Administrative Templates

Select System

Select Ctrl+Alt+Delete options

Select Remove Task Manager

Double-click the Remove Task Manager option

And as I mentioned above, since the policy is Remove Task Manager, by disabling the policy, you are enabling the Task Manager.

Hive: HKEY_CURRENT_USER

Key: Software\Microsoft\Windows\CurrentVersion\Policies\System

Name: DisableTaskMgr

Type: REG_DWORD

Value: 1=Enablethis key, that is DISABLE TaskManager

Value: 0=Disablethis key, that is Don't Disable, Enable TaskManager

http://www.dougknox.com/xp/utils/xp_taskmgrenab.htm