Others may know better than me, but I know of no "legal" free source of Win 7 installation media. There is a source you can use to create an Windows 7 Repair CD, but that can not be used to install Win 7.
Unless you removed it, your old hard drive should have a Recovery partition on it containing a compressed Windows 7 image -- which you could then use to restore your PC to its original state, and use the product key on the case to activate it.
If you removed the Recovery partition, and did not make a restoration DVD or CDs from that, you're basically out of luck at this point as, short of acquiring an Windows 7 installation DVD, you're not going to be able to get Windows 7 reinstalled.