They list "Idle" as a process but it's really just a No Operation the computer does to keep from losing its mind. If you add Idle and all the others it should always total 100%.
edit: otoh I guess there's a setting to "show processes from all users" in Task Manager. If it's not enabled then it may only show your processes and services. Just google terms: Windows Seven Task Manager ... and you'll get a bunch of hits with various aspects covered.
Also you can get free 3rd party Task Managers. One I like is DTaskManager