For Windows the majority of the issues tend not to be the operating system itself but the drivers and applications. That's quality control that Microsoft has tried to improve but it's still up to the component manufacturers and software developers to meet those standards. That applies to both built from scratch and pre-built machines
I agree with this. You're right that I can't blame Windows for not working well with Avid and other apps, but the fact the fact is that Windows just doesn't cut it in todays world. No one wants to crank start their car just like no one wants to contact four different vendors because their GPU drivers don't work.
Those that build their own machine can do so, and those that want to be up and running in no time flat can do so as well. On the same note though, no one can over look the failure of MS and Vista. Their blatant disregard for consistency and compatibility is another reason for MS not being the company it used to be.