Hi,
This issue is mentioned in following article:
"Task Manager has been disabled by your administrator" error message
http://support.microsoft.com/kb/555480
If you could enable Run with Win+R, try to add the registry keys with command line such as:
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d 0 /f
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