Windows 7 to New PC?

So if I were to build a rig, would I be able to plug in the HDD from my laptop, copy EVERYTHING onto the new HDD, and therefore have Windows 7 on both?

No, thats not how it works (the windows key is tied into the harddrive/motherboard). You can't just copy windows to a new harddrive and have the same product key tied to both harddrives

Nope. Won't work. You need to reinstall, and even then you can't have it installed on two machines at the same time with the same product key.