Technically (answer coming from Microsoft) is that an OEM copy can be supplied along with hardware. So in essence, a shop could provide an OEM copy of Windows XP along with just a Microsoft Mouse for example.
I used to work for a small computer company, and that's what Microsoft UK told us. Then again, Microsoft tend to be wrong a lot don't they.
Did you also know that when Microsoft provide bulk pre-activated XP cd's, all the key codes are the same (I've known 3 companies with EXACTLY the same key codes)