Sambariprasad

Why: if you didn't do it yourself, the news isn't good.
How to fix it: there are a couple of ways.
Let's look at this more closely.

The situation's pretty simple. If you right click on the clock in the Windows taskbar, the Task Manager item is grayed out:






Or, if you try to run taskmgr.exe manually, you get this message:

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.
If you have Windows XP Pro, there's a nifty user interface to directly edit the setting you want to repair. Click Start, then click Run and type in 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:

It should be enough to click on Not Configured and then OK and Task Manager is available once again.
If you don't have Windows XP Pro, then you'll need to edit the registry manually.
Click Start, then click Run, type in regedit, and click on OK.
Expand these registry keys in turn:

• HKEY_CURRENT_USER
  
• Software
  
• Microsoft
  
• Windows
  
• CurrentVersion
  
• Policies
  
• System
  

You should see something similar to this:

Right Click on the DisableTaskMgr item:

Click on Delete, confirm that you want to delete, and Task Manager should be available once again.
One thing to note is that it's quite possible that if you're in an environment managed by a centralized policy, these settings may have no effect. Your IT administrator may set things up so as to disable Task Manager, and other things as well at the domain level. You'll need to ask them if this is the case.