Freeware tools for gleaning system information abound, but few that I've come across are as compact or as complete (and powerful) as SIW (System Information for Windows).
SIW reports back on dozens of categories of information about a local machine, such as available hardware devices, its operating system, installed software and the network neighborhood. Since SIW is a standalone .EXE, and you don't need to install it, you can place it on a USB flash drive and run it anywhere -- which makes it an ideal addition to any administrator's on-the-go toolkit.
Here are some of the categories in which SIW gathers its data:
Open files: A complete manifest of all open files in the system along with which processes are holding them open. I've written before about how Windows has no built-in way to let you know who's got a file or removable drive that's locked for access; this is a good way to see, at a glance, who's blocking what.
Secrets: Use this one with care. It delves into many, many protected sections of the system and comes up with things like IE password-protected sites, AutoComplete form password data, product keys and other bits and pieces that are normally kept from prying eyes.
Protected files: This gives you a list of all files in the system that are classified as system-protected files. If you have doubts about a given component, you can call up the details about it here (although it doesn't tell you if the files in question have valid signatures).
Open ports: See all open network ports, including hosts, port state and the host application responsible for each.
Extended network information: Connects the computer to a number of external servers for gathering additional details about the network connection. Use it for doing extended troubleshooting when you're trying to determine if a computer has trouble interacting with outside networks.
Pipes: In this category, you'll see all open pipes in the system, including the number of instances for each.
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Serdar Yegulalp is editor of the Windows Power Users Newsletter. Check it out for the latest advice and musings on the world of Windows network administrators -- and please share your thoughts as well!