Most harmful viruses on the net disable the Task Manager and Registry editor making it difficult for you to deal with their infections. This makes almost impossible for us to kill the running malicious processes or delete the registry entries from the startup.
Always, running an anti-virus scan is the first solution in such case, but if still unsuccessful and you want to enable the Task manager manually, here are 2 simple methods to fix this error.
Goto Start > Run > in the box copy and paste this
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d 0 /f
and press OK.
Download and run this registry fix, from HERE
Off course, always restart PC or Re-login for the changes to take effect when u make changes in the registry.