I've been doing quite a bit of work in the IT shop on system imaging.
We use volume key's for our Microsoft Office and XP/2000 systems. So I don't usually have a real need to audit and recover these things before I create a image for deployment, or re-image a system.
However, when working on home systems for friends and family, I always try to capture the system OS key and any other software installation keys I might be able to grab before doing any serious work.
Nothing is a bigger bummer than having to reinstall an OS and then turn to the user and ask for their software key and get a blank look from them.
Despite the wealth of such programs now available on the web, there are a few main (freeware) utilities I use to recover Windows system keys.
All are tiny, portable and effective. Take your pick.
Be aware, however, that some anti-malware/anti-virus applications may tag them as hostile applications...since they can be used to recover potentially "secret" information from a system.
The Best Key Finders
Magical Jelly Bean Keyfinder v1.5 - (freeware) - It works on Windows 95, 98, ME, NT4, 2000, XP, Server 2003, Windows Vista, Office 97, Office XP, and Office 2003.
Magical Jelly bean Keyfinder v2.0 Beta 3 -(freeware) - It works on Windows 95, 98, ME, 2000, XP, Server 2003, Windows Vista, Office XP, Office 2003, and Office 2007.
ProduKey - Nirsoft - (freeware) - It works on Windows 95, 98, ME, 2000, XP, Server 2003, Windows Vista, Office XP, Office 2003, and Office 2007.
RockXP 4.0 - (freeware) - A longtime favorite of mine. Provides information on Microsoft Applications and Operating System product keys, Internet (dialup) connection password, Passport/MSN password, NT password Hashes, Protected storage passwords.
Here are some other key-finder programs. I haven't used these as much, but they do a pretty good job as well:
RJL Software Windows Product Key Viewer - (freeware) - Works with all Windows operating systems (95, 98, ME, NT, 2000, 2003, XP, Vista), 32-bit and 64-bit. Displays the full Windows version, service packs, product id, installation date and time, registered user, VMWare, organization and the current Windows uptime.
Jalapeño Keyfinder (formerly Keyfinder Thing) - (freeware/$) - the "demo" version is actually a full-featured application that scans and displays any of about 80 software titles. The $ version supports over 170 titles.
Keys and then some...
These three applications can provide application and system-key information. Additionally, they can provide a wealth of other hardware/software/driver/settings information about your system.
I find them useful to run when I am assessing a system for the first time.
WinAudit v2.26 - Free Computer Audit Software - (freeware) - great and fast program to inventory a system (hardware, network settings, security, configurations, licenses, etc.
SIW | System Information for Windows - (freeware) - Probably my all-time-favorite program in this class of applications. Wicked-fast and deeply helpful. Provides detailed information on software, hardware, network, tools, as well as some real-time monitoring functions. Supported platforms include: Microsoft Windows 98/Me/NT4/2000/XP/Server 2003/Media Center/Tablet PC/Windows 2003 Server R2/Windows Server 2003 x64/Windows XP x64/Windows Vista/ Windows Vista x64 / Windows Server 2008 / WinRE / Bart PE / Winternals ERD Commander
Belarc Advisor - Free Personal PC Audit - (freeware-personal use only) - Really popular and reliable system auditing tool. It locates and displays a profile of your system including installed software and hardware, missing Microsoft hotfixes, anti-virus status, CIS (Center for Internet Security) benchmarks.
Just don't be a bone-head and like some noobies, post your keys or audit findings on the web or in a public-web-accessible location. Protect your software keys like your passwords.