If your Task Manager is Disabled

Main

1) Click "Start" Button
2) Select "Run"
3) Type "regedit"

4)Navigate HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System
5) After Point System Check in the right pane for Disabletaskmgr, if you find it double click and change the D-word Valu to zero(0)
6) Close the regedit and restart system
7) Last but not Least When ever if u find any errors like this, That means Your System is infected with Virus. Better Update your anti virus software.

OR

Method 1 - Using the Group Policy Editor in Windows XP Professional

1. Click Start, Run, type gpedit.msc and click OK.
2. Under User Configuration, Click on the plus (+) next to Administrative Templates
3. Click on the plus (+) next tSystem, then click on Ctrl+Alt+Delete Options
4. Find Remove Task Manager in the right-hand pane and double click on it
5. Choose the option "Not Configured"  and click Ok. 
6. Close the Group Policy Window

Method 2:  Change the Task Manager Option through the Run line

1. Click on Start, Run and type the following command exactly and press Enter

REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d 0 /f

Method 3: Change Task Manager through a Registry REG file

1. Click on Start, Run, and type Notepad and press Enter
2. Copy and paste the information between the dotted lines into Notepad and save it to your desktop as taskmanager.reg

------------------------------------
Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System]
"DisableTaskMgr"=dword:00000000
-------------------------------------

      3. Double click on the taskmanager.reg file to enter the information into the Windows registry

Method 4: Delete the restriction in the registry manually

1. Click on Start, Run, and type REGEDIT and press Enter
2. Navigate to the following branch
   HKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Policies\ System
3. In the right pane, find and delete the value named DisableTaskMgr
4. Close the registry editor