As Droidln.net said, you need a family pack in order to this legally.
The other options to you are a fully packaged product - install on as many machines as you like - but only allowed to be on one machine at any one time. Or you can use OEM, but if you plan on getting a new pc (you are allowed to upgrade an existing one), the licence will die with that machine.
But, for any standard licence other than family pack, it is not legal and you will be breaking the EULA.
Lastly, Microsoft do have various licensing schemes that do allow a single serial key for multiple uses - however, these are not really designed for home users and can get very complicated!