**Edit** Note, please take this at face value for what it's worth (possibly nothing) Idon't want to get into an argument about nit picking detail or opinion.
OK, seeing as we're going through all this I thought I'd add a bit of background info which might help with some perspective on why OEM is what it is. Brace yourselves, this could be a long ramble
Microsoft (along with other software publishers) released Original Equipment Manufacturer (OEM) versions of Windows (and Office, DOS, Encata etc) with a view to it only being preinstalled and shipped on complete system by the normal OEMs or system builders (i.e. Dell, IBM, Compaq, OCUK etc). Originaly this was done by "licensing" the manufacturers to pre install the MS products on shiping systems.
The OEM versions were designed to be cheaper because they were going to be mass pre installed on systems. They therefore were licensed only for the machine they shipped with for the life of that machine (hence why you have the COA sticker stuck on a case somewhere). In addition the burden of support lay with the OEM rather than Microsoft (this is still the case, if you install an OEM copy of Windows you are soley responsible for the support of that install with no free support access to Microsoft beyond the web and any private support relationship you may have with MS as a Partner or System Builder). This suited the OEMs as although they undertook to support the OS as well as the hardware it helped them cement an ongoing relationship with thier customer. Obviously the OEM versions also tended to not have manuals etc.
This was back at a time where very few end users upgraded PCs. History until that point suggested that PCs would be a commodity and would be used for 3-5 years and then replaced with a new system (companies tend to replace hardware every three years as that is the time they tend to get written off against tax).
Some of the more "enterprising" OEMs (and I don't mean any of the above) realised they could sell the OEM versions of the software along with an item of hardware to end users to supliment income and get around the OEM restrictions. This was never the original intention of the software and shows why, in part the EULA isn't paricularly end user friendly, it was never intended to be used by end users in that way. Last year Microsoft accepted that it was too late to bolt the stable door and accepted that OEMs would continue to sell OEM Windows and so removed the requirement for to to be supplied as part of a hardware transaction.
However, the limitations in term of transferability etc remain. Like Burnsy I suspect that if MS were forced into a corner over the tranferability they would be faced with two products, retail and OEM which were in effect exactly the same except one was cheaper. In this case I would expect MS to have to rethink the business model. Now that could mean they dropped retail pricing and just issued one version at OEM price. That seems unlikely though. The other option would be to change the OEM product so that it was no longer available to end users by, for example having a new agreement with the OEMs that to be eligable to buy OEM editions at that price they would have to undertake to only supply it on complete systems (complete could be defined as a pre assembled case, PSU, motherboard, processor and RAM for example).
Of course the last bit is purely conjecture and I doubt MS and it's partners would ever go down that path unless OEM misuse became a real problem, which I can't imagine it would. Still, just a bit of second guessing fun on my part
Hope that's a least a little useful in giving a bit of background around why the OEM restrictions are what they are and why they are there.
Ath