Head over to the Mystery Solved site of microsoft.com and click on the usb stick on the right where it says Get Yours. You need to sign in with passport.net and answer the following questions:
Q1. How many ways are there to obtain a full Microsoft® Windows® Desktop license?
Q2: Volume License Agreements cover Windows Desktop operating system upgrades only.
Q3: OEM operating system licenses are non-transferable.
Q4: The most cost-effective way to acquire an initial, full underlying Windows Desktop license is preinstalled.
It will take between 6 and 8 weeks before you get the stick, but free is free, right ?