I've always considered myself a hardware guy. Even my job is a hardware engineer. I can say that I never built a "gaming" PC -- I've built PCs for productivity, entertainment, and later, video editing. Now I just like building retro systems for the experimentation and challenge of hunting down hardware and software, getting things to work, and simply trying things now that I couldn't when I was younger (and didn't have as much money)!
I run my Windows 98 PC on a widescreen LCD monitor (it looks beautiful, by the way), I had a wireless keyboard and mouse on the system as well. I just don't want to deal with CRTs -- the were big, heavy, and had limited resolution then, and still do now. Plus, I prefer the lower power consumption, space savings, and just plain able to get a big cheap one at Goodwill.
I'm not into MIDI sounds and can care less about the quality of the sound card. That being said, I'm an audiophile with a large record collection and a stereo system with multiple turntables and vintage audio gear (another one of my many hobbies). I even prefer onboard sound chips! I have SoundBlaster 16 in my retro 486 PC and a SoundBlaster Live in my Windows 98 PC, but as long as the games work and the sound plays, it works for me.
I consider anything that does not run a current operating system to be "retro" -- and by "current", I mean one that is still receiving support from the source. At this point in time, I would say the cutoff is late Pentium 4s that can still run Windows 10 and be useful for e-mail, internet browsing, and limited HD video streaming. While I grew up mainly in the era of the original Pentium, my first computers were an 8086 with DOS 3.1, a 286 with Windows 3.1, and later a 386-SX with a 28.8 modem for JUNO e-mail, I could consider a new-ish computer retro if it won't do what I want a computer to do today.