Belarc Advisor ( is free for personal use only.  It is designed to provide a detailed report on your software and hardware, but can also be used to find product keys used for software on the system. 

Windows Repair (All In One) ( is free to use.  I first found out about it when I saw a technician from Microsoft install and run it while trying to fix Windows Update problems on a support call.  The program can help fix a large majority of known Windows problems.