Just buy the OEM version. Even Microsoft themselves can't agree on whether it's acceptable or not (http://www.zdnet.com/blog/bott/is-it-ok-to-use-oem-windows-on-your-own-pc-dont-ask-microsoft/1561) but as far as I'm concerned after interpreting the license as best as I could (IANAL etc), it's absolutely fine.
The crap about selling it with a fully assembled system is for when the EULA takes effect: which is when it's been opened. While it remains unopened, PC enthusiast vendors are just selling the package and the agreement doesn't apply yet. It applies to the person who opens it, and if you're the "system builder" (in this case all you have to do to "build" it is er, install the OS?) building for yourself then don't worry about anything it says about the agreement between the builder and the buyer. You can't sue *yourself* :)
The people who speak for Microsoft are mostly call centre droids who don't really know either way.