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

Task Manager has been disabled by your administrator

This error is caused by restrictions placed in Registry. DisableTaskMgr value is set to 1. To enable Task Manager, try any of these methods:

Resolution

Method 1

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

Method 2

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

Method 3

• Open Registry Editor (Regedit.exe) and navigate to:
HKCU \ Software \ Microsoft \ Windows \ CurrentVersion \ Policies\ System
• In the right-pane, double-click DisableTaskMgr and set it’s data to 0
Method 4: Using Group Policy Editor – for Windows XP Professional
• Click Start, Run, type gpedit.msc and click OK.
• Navigate to this path
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.