If you have problem running your task manager because it was disabled by the administrator as the error say, you might have gotten your computer infected with a virus/worm.

There are two easy ways to fixed it, one is through group policy editor and the other one is through registry.

Using the group policy editor (gpedit).

1. Click on start then click run, type in gpedit.msc and click ok button.

2. Once the group policy editor opens, click on user configuration then click on system.

3. Look for the crtl-atl-del option and double click on remove task manager.

4. Select the disabled option and then click apply button.

Using the Registry Editor

1. Click on start then click run, type in regedit and click ok button.

2. Go to this registry key HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System

3. Modify or Create the value data type DWORD and the value name is DisableTaskMgr.

4. Set the value data to 0 and exit.

Reboot your computer for the changes to take effect.

In any case the above procedures fail then you may use a third party program called HijackThis which you can download here.