Have you by any chance added a "shortcut" to something using the keys "Ctl-Alt-Delete?" Since it's the system key-set to open task manager, it might do that even if you've set the same keys to do something else(?). (I think the "Ctl-Alt-Del" is generally "hard programed" in BIOS on most machines.)
Right clicking on a blank space on the Task Bar (bottom of the screed) and clicking on Task Manager (or hitting "k") will also open it.
The Command (DOS prompt) utility "schtasks" allows you to schedule actions that the computer does automatically. You can set it to repeat as often/frequently as desired, at fixed time intervals or when "something" happens. A slightly botched entry in the scheduler could open Task Manager repeatedly, one supposes. If this utility has been botched, it should keep a log of what it's done, or tried to do at Systemroot\SchedLgU.txt where Systemroot\ is your system drive, probably C:\. (May be a hidden/system file, and may require Administrator login to open?)
Start|Run, type "Command" (w/o the quotes) and click Go or hit Enter will get the DOS window. Typing "schtasks" and Enter will show what's been scheduled. (You may have to logged on with Administrator privies to get a result.)
At the Command prompt, type schtasks /? for help on the command; but be careful as it's pretty powerful.
(Type "Exit" and Enter to close the Command window, or course.)
I haven't heard of any malware doing what's described, but it might be "reasonable" that a spyware bot might want to know what programs you have open, and might open a new Task Manager each time it tries to call home(?).
Suggest the usual, update and scan with your AntiVirus, and with Ad-Aware and Spybot if you don't have other better(?) anti-malware programs.