You can replace all but the motherboard without having to purchase a new license. However, if the motherboard should fail, you can replace with the same make/model. The license is tied to the motherboard (MS considers the motherboard to be the computer), not any other piece of hardware. You can replace/upgrade everything but the motherboard. The one exception to the motherboard rule is in the case where the same make/model is no longer available, you may replace with a comparable board. Upgrading a P4 to an i7 (as an extreme case) is not comparable. While Microsoft does let some slip by they are by no means obligated to allow you to use an already activated key on a new PC.