The OEM states that it can only be used on that motherboard, as soon as that changes, the install becomes invalid till you put in a new product key. OEM product keys are one-time use, that's why their cheaper than retail.
Retail Windows you can install on multiple machines without issue.
If you were to buy this, you get the Windows disc and a product key that is only good for one use, and it knows this by binding itself to the motherboard. If you change the mobo (or move the HDD to another system), it will invalidate the install. If you try to use the product key on another build, when windows verify's itself, it will see that its already been used. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
If you re-install windows with the same motherboard, I think it would be fine. It will recognize its the same system and validate properly.
I think the keys attached to the bottom of a dell or hp laptop, the sticker under it or even one attached to a desktop are different then. Because I used a Win7 home premium key from a hp laptop on my dell laptop and I believe these specific keys are called OEM SLP keys and can be used with any oem brand/manufacturer.
I've also used a winxp pro key from my old dell desktop on other comps to. Never ran into any troubles.