Task Manager has been disabled by your administrator

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.