As far as I can tell the product doesn't get tied to your hardware, but cannot be actived a second time, unless you contact Microsoft.
I believe it has more to do with Microsoft Licensing (ie pricing ), all the versions are pretty much the same once installed.
What it all boils down to is that if you want to re-activate a DSP/OEM/AE product with different hardware you may need to phone Microsoft and tell them your original hardware failed (If you can live with telling a small little fib

)
Basically, Microsoft products are most commonly classified into four groups:
1) OEM - Original Equipment Manufacturer
According to the license agreement an OEM license may only be sold with a "qualifying hardware product" which basically means that the suppliers can decide which products they want to provide with OEM licenses. It's basically a tool to help promote brands and provide discounts to clients who make use of these brands. An example would be buying a new branded PC (HP, Mecer, Acer, whatever) and getting the OS a bit cheaper.
2) DSP - Delivery Service Partner
This category is slightly more expensive than the OEM version and basically targets clients who don't necesarilly want to purchase brand name equipment, but rather assemble PCs from components.
(Both DSP and OEM are intended to be non transferable, typically requiring you to phone Microsoft with a valid reason (Hardware failure most commonly) if you wish to re-activate the OS on a different machine)
3) AE - Academic Edition
Discounted versions of Microsoft products aimed at students and acedemics, where the business components of the software are typically removed (Example: Outlook removed from Office AE), and users allowed to install and activate the product on multiple machines (typically 3)
4) Retail
Targeted at the end user, usually ALOT more expensive than DSP/OEM. I heard once that they allow you to activate the product on a different machine, as long as you remove the product from the original machine, but don't quote me on that.
I have found that most suppliers in SA are not all that strict on the licencing issues and will pretty much sell you whatever license you request. Might bite you in the but if the BSA ever came round, which is not all that likely with home users, unless you REALLY give them a reason.