Usually this caused by a virus or your administrator disabled access to it.

Symptoms:
[*] A grayed out task manager when right click at [url=http://en.wikipedia.org/wiki/Taskbar]Systray[/url] clock
[*] If you run taskmgr.exe directly via “Start Menu >> Run” or command line it will give you the following message:
Task manager has been disabled by the administrator

Fix:
[*] Start Menu > Run
[*] We need to access Group Policy Editor, type:
gpedit.msc
[*] Navigate to: User Configuration > Administrative Templates > System > Ctrl+Alt+Del Options
[*] Double click “Remove Task Manager” and change its setting to “Disabled” and click Apply
[*] Double click “Remove Task Manager” and change its setting to “Not Configured” and click Apply and OK
[*] Restart the computer for the changes to take effect.

Good luck!