It's a very interesting question. During or just before Vista, if you wanted to get Vista/XP for your computer, you could buy it online, preferably with your new computer hardware, and install an OEM version. You can still buy heaps of OEM versions to this day. It could be any version, and 32-bit or 64-bit if you wanted it to be. It was bound to the motherboard, and would die when the computer (your motherboard) died or was trashed.

But I'm not sure in this age of Windows 7.

What if you wanted to install it onto your old computer (doesn't matter what was on it) to make it Windows 7?
What if you wanted to install it onto a new custom made build?

Because as far as I am aware, as long as you are okay with it being bound to the motherboard for the life of your computer, and you become your own tech support, going OEM is the cheapest way to go. We don't need to go through a systems builder package at all. We just install it, select the right edition the key relates to, and then activate it using the key. I'm sure we can handle driver instillations as well.

So at what point are Microsoft going to care about any of the many hobbiest out there buying OEM copies when it might be against their rules?

What Microsoft won't tell you about Windows 7 licensing | ZDNet
Is it OK to use OEM Windows on your own PC? Don't ask Microsoft | ZDNet