Sometimes, when a computer has been infected with viruses or spyware, the task manager will be disable.
To solve this problem, follow the steps below:
1. Click on Start --> Run
2. Key in REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d 0 /f and press enter
Make sure you run antivirus to check on your computer to ensure that your computer is free from viruses.