Enabling Task Manager

Cause & solution for it
A.Task manager has been disabled by your somebody in administator account using registry editing(regedit)or group policy(gpedit)

Solution1 using group policy
a Go to Start then Run then type Gpedit.msc and press Enter button.
b. click on User Configuration then Administrative Templates then System then Ctrl+Alt+Del Options
c click on remove task manager
d set to disable to enable task manager.

2. using registry editing
a.Go to Start then Run then type regedit
b.double click HKEY_CURRENT_USER then Software\Microsoft\Windows\CurrentVersion\Policies\System then DisableTaskMgr then REG_DWORD then put o as value to enable task manager.
B.It may have been disabled by virus,malwares,spywares etc
Install a good antivirus and scan your pc and follow the step above steps.You can also get free version of antivirus.

Some of the links of antivirus are
www.avast.com/free-antivirus-download
www.free-av.com
free.avg.com
www.bitdefender.com/PRODUCT-14-en--BitDefender-Free-Edition.html
www.kaspersky.com/virusscanner