I had this happen to a machine of my own recently, after it copped a virus.
Some persistent work and scanning with up to date antivirus, Adaware, Spybot and Trojan Remover, together with several reboots, eventually eliminated the problem. Along with the 'Task Manager' problem, I'd also lost 'Device Manager', 'Add/Remove Programs' and several other functions. All came good with persistence.
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