Unless the administrator did that (did you ask him?) it's a sign of malware.
If your current antivirus can't find it, I'd try MBAM (Malwarebytes' Antimalware) or a bootable anti-virus disk, as discussed in http://www.howtogeek.com/175234/how-to-use-an-antivirus-boot-disc-or-usb-drive-to-ensure-your-computer-is-clean/
There are more than he mentions, but one of them already might do the trick.
Kees
Hi,
When I press CTR+ALT+DEL, showing Task Manager has been disabled by your administrator.

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