"If I remember rightly previously joe public were allowed to buy the OEM version"

I've historically tried to use official/supported/legal versions of all of my applications. Windows was one of those and I actually read parts of the OEM EULA at one point.

The following link sums up the use of an OEM license well:

http://www.microsoft.com/oem/en/licensing/sblicensing/Pages/...

The most important part is here:

OEM System Builder: Must be preinstalled on a PC and sold to another unrelated party. vs

Full Packaged Retail Product: Is intended to be re-sold separately from a PC for installation by the end user.

Disclosure: Microsoft Employee. I had the obsession to have properly licensed software on my machine long before that.