Task manager disabled (XP/Vista/Win7)

is is a policy that can be modified by Gpedit. The key (such as the ones of gpedit) is

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System

In the right pane we will find a value named

DisableTaskMgr

We only have to remove or set it to "0" (zero) to enable our task manager again.

Another possibility is to modify the key from command screen (start/run/“cmd.exe"). For this task you have to type in this command (it is a single line):

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

This will enable the task manager.

You can also do all this in an automatic way by downloading this script