In Windows some spyware programs do disable the taskbar to prevent the closing of the spyware process by Advanced Users through TaskManager.

Maybe check with Ad-aware and an Antivirus and try this to enable task manager or You can do it in Another way :

Click Start
Click Run
Enter gpedit.msc in the Open box and click OK
click ok to bypass the UAC prompt
In the Group Policy settings window
Select User Configuration
Select Administrative Templates
Select System
Select Ctrl+Alt+Delete options
Select Remove Task Manager
Double-click the Remove Task Manager option
Since the policy is Remove Task Manager, by disabling the policy, you are enabling the Task Manager.