Task Manager disabled by malware??
Save this text (below) into a text file, then rename to somename.reg double click and agree to changes.
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System]
"DisableTaskMgr"=dword:00000000
If it enables task manager, you need to do some serious malware scanning.