Summary: Task manager can be disabled manually, but more commonly it's disabled by a virus. It's easy to re-enable once you're virus-free.
My task manager has a "task manager has been disabled by the administrator" message. How did this happen and how can I correct this?
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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.
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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.
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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 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.
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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.
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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.
Related:
Where is Task Manager? Task Manager can be found or started in a couple different places in Windows.
Why does Task Manager suddenly have no Menu or Tabs? If Windows Task Manager doesn't have Menu or Tabs, then it is probably running in what's called "Tiny Footprint Mode".
What's the best registry cleaner? Registry cleaners are widely available. When do you need one? Why? In general, a registry cleaner isn't needed and when it is it shouldn't be used frequently.
Microsoft - "Task Manager has been disabled by your administrator" error message Knowledgebase article
Article C3088 - July 16, 2007
i cant open task manager using CTR+ALT_Shift. I just scaned with Malwarebytes and it found 2 items infected. I removed them, then RUN gpedit and hta Ctl+shift+Alt was set at Unconfigure,but i still CANT open tasmngr using the keys,Note: i can open it by right click on the clock, i appreciate any help you can give me
Posted by: Long ho at February 1, 2010 2:26 AMTask Manager will NEVER open with CTR+ALT+Shift. The key combination is CTR+ALT+DEL!
Posted by: Jaime at February 1, 2010 9:43 AMMy Start button and Windows taskbar doesn't show on my screen, all my icons are gone. When I press CTR+ALT+DEL is indicates "Task Manager has been disabled by your administrator." How can I follow the steps above if I can't even get to the Start menu or clock in the Windows taskbar? I have Windows XP.
Posted by: L McKinley at February 3, 2010 5:29 AMI have the same problems as L McKinley with xp pro. If i open in safe mode and click on the administratior is says loading settings and then goes right into saving settings. If i open normally i get just desktop background. Is there a way to get into safe mode so i can try some of these options?
Posted by: jacob at February 3, 2010 9:33 AMWell that makes at least 3 of us. I also have the same problem. No task manage, no icons no start menu. Just a blank background screen. What more, I can't boot into safe mode or any other mode for that mater other than normal boot or last good boot.
Posted by: Adam at February 3, 2010 11:40 PMjust before this happened I got a system will shut down message on the screen. then on the reboot all was gone. please help thank adam