If you don't preinstall operating systems, you don't have to stock computers with different operating systems - you just stock OS-less computers and have boxes of CDs (that take up next to no space) with different OSs. Or better yet - you download the newest updated version and give it to the user on a CD or flash card.
As for hardware being made for Windows, the only way this is visible is that the vendors supply drivers to Windows. Most devices these days use standard interfaces (USB, ATAPI, etc.) that aren't specifically targeted to Windows. And if it was not for Windows dominance, all devices would now carry their own drivers in bytecode on a flash ROM and upload it to the host when plugged in. No more messing with drivers etc. - you get real plug and play.