Hello,
First thing I would do is search for the file taskmgr.exe in the directory c:\windows\system32\ to see if it exists...
If taskmgr.exe exists in that directory, and you can run taskmgr.exe manually, but still not by ctrl+alt+del, then search the directory c:\windows\system32\ to see if the file taskmgr.com exists, and if so, delete it.