Many times when working on a computer that has been infected with a virus, trojan, or piece of spyware you see Task Manager being disabled.A Restriction has been placed on the user through viruses/trojans/spywares to not allow user to run Task Manager.Listed below you will find the many ways to re-enable Task Manager along with an automatic method that works wonders.
To open the Task Manager, you normally would do one of the following

Press CTRL-ALT-DEL on the keyboard.
Press CTRL-SHIFT-ESC on the keyboard.
Right-click on a blank area on the start bar and choose Task Manager.
Click on Start, Run and type TASKMGR in the run box and press Enter.

Sometimes instead of Task Manager opening you’ll see the following screen.

In these cases, you’ll have to follow the methods below to re-enable access to the Task Manager.

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

• Click Start, Run, type gpedit.msc and click OK.

• Under User Configuration, Click on the plus (+) next to Administrative Templates

• Click on the plus (+) next tSystem, then click on Ctrl+Alt+Delete Options

• Find Remove Task Manager in the right-hand pane and double click on it

• Choose the option “Not Configured” and click Ok.

• Close the Group Policy Window

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

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

• Now Type “REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d 0 /f ” (Without Quotes)

Method 3: Change Task Manager through a Registry REG file

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

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

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Method 4: Delete the restriction in the registry manually

• Click on Start, Run, and type REGEDIT and press Enter

• Navigate to the following branchHKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Policies\ System

• In the right pane, find and delete the value named DisableTaskMgr

• Close the registry editor.

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