If you want to setup your computer system again you need to have the licenses and serial numbers at hand for all the software programs that you have purchased and registered. This does include the Windows product key but also other serial numbers for applications like Nero, Office, VMWare and pretty much every other application.
Instead of searching for the keys in your emails, manuals and recipes you could use another approach. License Crawler is a sweet little application that scans the Windows Registry for Windows product keys and other serial numbers and licenses.

The License Crawler can discover application product keys and other serial numbers or licenses very quickly and supports all versions of Windows operating system from Windows 95 to Windows Vista.

It is a portable application that can be run from any location and would be a perfect fit for an USB stick. Asking clients to find serial and license keys is often very frustrating. Instead, tech support can simply run License Crawler without having to interact with the client at all.