Belarc Advisor (free) is one of the handiest programs going, especially for IT types. This program creates an HTML report that not only tells you everything you could possibly want to know about the hardware on your system (except perhaps why it doesn't work correctly), but also what you might need to know about the software that's installed. Gathered info includes motherboard type and revision, CPU and GPU info, drive space, Microsoft hotfixes, operating system revision, and Web browser vulnerabilities, as well as third-party software installs.
My favorite Belarc Advisor feature by far is its software key retrieval. I can't tell you the number of times I've had to rebuild an operating system installation for someone who never had lost all their software discs and documentation. It's easy to download replacement software, but it's often useless without the installation keys--a lot of it won't install or will be limited to demo status.
If you can boot into the OS at all, Belarc Advisor will retrieve the keys for software installed on the system for you. Even if you have all your original install discs or info, having these keys gathered together into one document can be very handy. When I install software or optimize a system for an IT customer, I always subsequently run Belarc Advisor so I have the information on hand for them in the future.
Belarc Advisor is of course a leader item for the company's pay software for managing LANs and the like--but whatever their reason for publishing this little goodie, it's fine by me. My only gripe is that it must be installed on the PC. A portable version would be nice, as would a version that would scan the registry of unbootable systems.