hmm i think you need to edit your windows regirsty manually.

• First of All go to start menu, and click "run".
• Than type "regedit" and click ok
• You will get a registry editor
• Than find HKEY_CURRENT_USER\Software\Microsoft\Windows \Currentversion\Policies\System\
• There you may find "DisableTaskmgr"
• Right click on that, and delete it.

I hope this ll solve your problem.