Belarc Advisor (www.belarc.com)
The Belarc Advisor builds a detailed profile of your installed software and hardware, missing Microsoft hotfixes, anti-virus status, CIS (Center for Internet Security) benchmarks, and displays the results in your Web browser.

Process Explorer for Windows (www.microsoft.com)
Process Explorer shows you information about which processes are open and running. This can be useful for diagnosing problems with processes that may be slowing down your PC.

SIW (www.gtopala.com)
SIW is an advanced System Information for Windows tool that gathers detailed information about your system properties and settings and displays it in an extremely comprehensible manner.

Tweak UI (www.microsoft.com)
This PowerToy gives you access to system settings that are not exposed in the Windows XP default user interface, including mouse settings, Explorer settings, taskbar settings, and more. Unfortunately, this program is not compatible with Windows Vista.

WinDirStat (www.windirstat.info)
WinDirStat is a disk usage statistics viewer and cleanup tool for Microsoft Windows. This software gives you a graphical look at the files on your hard drive, letting you see which files and file types are taking up the most space.