In order to restore taskmanager first make sure the system is free from anytype of viruses, trojans etc. Windows XP Proffessional users follow the steps as shown rest can see at the end for other Windows versions.
Step 1: Click Start -> Run and type gpedit.msc and click on Ok.
Step 2: After clickin on Ok, a Group Policy window will be opened.
Step 3: In the left pane, expand in turn
User Configuration -> Administrative Templates -> System -> Ctrl+Alt+Del Options
then you'll see something like this on the right pane.
Step 4: Double click on Remove Task Manager to change its setting.
Step 5: Click on Not Configured and then click on Ok and restart your system.
"For users who don't have Windows XP, can follow the steps shown below"
Step 1: Click Start -> Run and type regedit and click on Ok.
Step 2: In the left pane, expand in turn,
My Computer -> HKEY_CURRENT_USER -> Microsoft -> Windows ->CurrentVersion -> Policies -> System and you'll see something like this
Step 3: In the right pane right click DisableTaskMgr and click on Delete and confirm that you want to delete as shown.
Step 4: Restart your computer.