Most viruses when they infect a computer disable Task Manager, Registry Editor and Folder Option so that they could not be removed and you get a message like this when trying to access them:
Task Manager
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Se7en Task Manager Modder is a portable application that allows you to change the color of Windows 7 Task Manager.
Out of the box, Task Manger provides some useful information in the Process tab for getting a quick snapshot of performance and system activity. But it can display much more than just basic CPU and Memory information.