Process Explorer is a system resources monitoring tool for Windows operating systems. It follows the general lead of the built-in Windows Task Manager tool, but extends the feature set greatly.

The OneCare Removal Tool is a program created by the Microsoft OneCare product team that can be used to remove OneCare from your computer in the event that it is not uninstalling properly.

The Microsoft Security Essentials Removal Tool can be used to remove all traces of Microsoft Security Essentials from your computer in the event that you are unable to remove it normally via the Windows control panel.

RootkitRevealer is a rootkit scanner from Microsoft Sysinternals.  This program will search for user-mode or kernel-mode rootkits and list any API discrepancies that are found.

TCPView is a handy little program that allows you to see the network activity on your computer.  When run, you will be able to see what remote devices or computers that your individual programs are communicating with as well as what programs are waiting for connections.