By the mid to late 90s, I don't think DIY computers would have made much of a dent compared to prebuilt computers, especially since most of them would have ended up running Windows.

I wasn't able to find web usage stats that are very old. http://www.geek.com/wp-content/uploads/2009/05/os_marketshar... is the oldest usage-based data I could find, which puts Windows up at around 97% in 2005. I can't imagine it was much lower in the late 90s, given that Macs of that era were pretty shitty and even less people used Linux back then.