HOW TO ENABLE TASK MANAGER IF DISABLED

So, how did this happen?

As we'll see in a moment, there are settings you can access as an administrator that would allow you to disable and enable Task Manager in this way.
You didn't do that, you say?
Then a virus probably did.
Disabling Task Manager is one way viruses try to make it harder for you to deal with their infections. Before proceeding any further, you should run a complete and up-to-date anti-virus scan of your machine. It's possible, perhaps even likely, that you've been infected.
Once you come back virus-free, you can proceed with the fix.
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If you have Windows XP Pro, there's a nifty user interface to directly edit the setting you want to repair. Click Start, then run and type gpedit.msc:-

And press OK. This runs the Group Policy Editor.

Once in the Group Policy Editor, expand in turn:

User Configuration
Administrative Templates
System
Ctrl+Alt+Del Options
You should see something much like this: 


Double click on Remove Task Manager to change its setting:

THATS ALL 

Source:   extreme tips