re-enable task manager

Click Start, Run and copy/paste this command:
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d 0 /f
Click OK.
If you still cannot access the Task Manager try the second method:
1. Click Start, Run and write: regedit
2. Go to the following:
HKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Policies\ System
3. In the right-panel, right click on DisableTaskMgr, click on Modify and change the Value to 0
4. OK
4. Close the window
Now try CTRL+ALT+DEL. It should open the Task Manager without any message :)