The best secure place to get a copy of Windows is from Microsoft directly, it'll be expensive but worth it :)
OEM activation has a few versions, FYI OEM stands for Original Equipment Manufacturer a.k.a the manufacturer.
1. You have the OEM SLP what stands for System Locked Preinstallation, these are used only once as they are bound to the hardware and flashed in the BIOS, these kind are used for Windows that comes preinstalled on laptops.
A OEM SLP can be called a hardware key, when reinstalling Windows a key is not required so it can be skipped during installation. Normally with a OEM SLP activation you never have to enter a key.
2. There is also the NSLP one, this is a Non System Locked Preinstallation, this one does need to be entered when while installing a key is required.
This one can be seen as a retail key, but it is distributed and supported by the OEM. As only a motherboard change is seen as a new system a phone call to Microsoft is required for activating this key when changing the motherboard. Microsoft will activate the key ONLY when the replacement is because of a problem with the motherboard like a failing or faulty one, not when you want to upgrade while there is nothing wrong with the current one.
3. There is also the OEM COA, COA stands for Certificate of Authentication. The OEM:COA is normally provided via a COA sticker pasted on the casing (side or bottom body) of new desktop, notebook, laptop, netbook or nettop PC, which has already been activated with a pre-installed OEM:SLP key by the OEM.
A retail key doesn't really have specific requirements that I'm aware of, I did not read the EULA (End User License Agreement) but I'm aware of a few things by reading many threads about activation.
A retail key can be installed on any system without reactivation through phone call or something, I believe the only requirement is that the key needs to be uninstalled from the previous system to be used again.
