The short answer
Retail follows you: you can generally move it to a new PC once the old one stops using it, subject to Microsoft's licensing and activation rules. OEM stays with the first PC it activates and costs less. If you'll keep the PC as it is, OEM is good value. If you rebuild or replace PCs, retail saves you buying again.
Side by side
| Retail | OEM | |
|---|---|---|
| Windows features | Identical | Identical |
| Devices | One at a time | One, permanently |
| Move to a new PC | Generally, yes | No |
| After a motherboard swap | Can generally be reactivated with the same key | Usually needs a new license |
| Reinstall on the same PC | Reactivates | Reactivates |
| License support | Microsoft | System builder |
Why the motherboard matters
After activation, Windows records a digital license tied to your hardware, identified mainly by the motherboard. Changing a drive, memory or graphics card usually doesn't disturb activation. Changing the motherboard looks like a different computer. A retail license can generally be reactivated on that "new" computer, subject to Microsoft's licensing and activation rules; an OEM license generally can't. More in activation after hardware changes.
What OEM "system builder" terms mean for you
Microsoft's OEM licenses are designed for companies and individuals who build PCs. The builder installs the license and is responsible for supporting it. If you install an OEM key on your own build, you take that role. Everyday use is unaffected; the practical effect is that you shouldn't expect Microsoft to help transfer an OEM license.
Which should you buy?
- Building a PC you'll upgrade over time: Pro Retail or Home Retail.
- Licensing a PC that's finished and will be retired as-is: Pro OEM.