Many years ago, when I was a poor university student (Uni is free in Oz, or at least learn now, pay later), I worked out that I could put together decent systems ($1200au) for about $200-300 cheaper than stores would charge for systems with similar specs, but had skimped on product quality and features across every component.
That $200-300 would buy a M$ OS .. but then .. that's $300 worth of beer ... or books ... or food ... or fees .. or clothes ... or ....
I've currently got Windows7 beta installed. The ultimate version is nice ... but .. the only reason I use it is for games. And Cedega seems to be doing a semi decent job of playing most of the games I use, and I do have a windows XP license that will play every games I have XP I already own ... and is more than enough for (current) games ... and cedega is only a few $$$ a year .. a huge difference to M$ OS's @ $250+ (oem versions) or $600 for uncrippled retail versions.
The Exodus that started with Vista will continue ... soon M$ will be releasing it's own customised version of linux - free ... and then once there is enough proprietary code included, and everyones switched to M$'s Free Linux, they'll start charging for Linux based OS's
All the OEM's seriously skimp on hardware quality (especially interchangeability), cooling, and other things which are essential to any PC enthusiast, but detrimental to future sales.
The biggest issue though, as a gamer, is the poor crop of dedicated video chips that OEM's use ... they generally use crippled second tier chips, which give you the performance of a budget/low-end discrete video card - at the price of 50% higher than if you bought a high performance discrete video card seperately.
And what's the deal with adding "Soundblaster Audigy Software" to the PC's .. cheap hardware sound output chips that are like the win-modems - no discrete sound processing ability?
Oems serve a purpose ... the "I'm too stupid to own a computer", and the "I need a typewriter for business" segments of the market need their sugar.