How can you possibly know what his work requirements are? If it's a work PC and he needs to join a domain, he will need more than "home premium," but the disclosed HW specs either way are fine for Win7, whether x64 or x86.

sixto, spdrun is halfway right about reusing the key, but you would have to re-register it with M$ as it will be associated with a different hardware profile.

If the PC is more than a few years old, I would DEFINITELY install a new hard drive in it rather than relying on the old one.

Also, note that most PCs you buy now are sporting Win8 of one flavor or another. Business-class PCs (that usually cost more) are available with Win7.

Thus, all the way around it makes more sense to me to swap your hard drive and install Win7 on it. Check pricewatch, this place has the full version of professional for $60: Microsoft Windows 7 Professional 64 bit Full Version OEM (Dell)

Good luck.