if you wondering from where the software installed on your computers are coming from, I suggest you to use an asset management tool.
you can found free tools on the BSA web site (http://www.bsaeurope.org/audit_2007/free_tools.php) or buy somes.
Microsoft is providing a tool to generate signatures for the products installed on a machine. You can then send this information to the user about his installed products.
Software asset management is a big thing as you need to control the whole machine park and understand the licencing model of each software vendors.
for example, if you are a developer using Microsoft products for the test, you can download the bits from MSDN, or your volume licensing web site with "production license" keys. This can be confusing some times but we have all to be aware of the property of the software we are using.
Don't hesitate to ping me if you need some help on the Microsoft developer tools.
Best regards and happy new year 2008