There are three types of licencing that Microsoft has for Windows:
1. OEM System Builder for distribution with new PCs. This is tied to the hardware. When you buy it, you get a COA sticker, which goes on the case... It is then tied to that system and cannot legally be moved though if you replace a motherboard you can call Microsoft and get it re-activated...
1b. OEM like Dell/HP. These are tied to specific hardware. i.e. Dell media will most likely not install on a Non-Dell system, so if you have a Dell laptop running Windows 7 Home Premium 64-Bit, you need: Dell Media for Win 7 Home Premium 64 bit and the COA on the computer to install. Again it is tied to the system. If you replace a motherboard it should re-activate without needing to call Microsoft provided Dell provides the motherboard... otherwise it won't work.
2. Retail. This is a boxed version of Windows like you would find in Walmart or Staples. It is NOT tied to a specific computer, but you can only run it on one computer per box/per key. If you wish to move the licence to a different computer you can, but you will need to decommission the coomputer running it i.e. Install Ubuntu
and contact Microsoft.
3. Volume Licencing, which is negotiated by the licencee for both price and volume. This can be installed on anything, generally does NOT have activation, and can take advantage of the key management server... It is NOT for "At-Home" customers. You keep track of the licencing yourself, you do audits for Microsoft, etc. It is the easiest, but there is a huge minimum i.e. You cannot only one or three...
Although it generally passes Genuine Validation no problem at all unless the key is blacklisted, it is still highly illegal to use it at home unless you are taking a computer home from work (i.e. a laptop).
You can look at the product ID. I am at work right now on my Widnows 7 PC, but to the right of me is my Windows XP PC on my desk. It's product ID looks like: XXXXX-640-XXXXXXX-XXXXX. If you EVER see a customer's PC with 640 in the product ID, it is most likely that a Volume Licence is being used illegally. In my case it is fine because we have 2000+ computers and a VLA with Microsoft.