yes I think I remember a friend playing Hl2 on w98 back then (with a p3, steam was very different in 2004), so a cracked vanilla copy should work, I think the same would apply to HL:S
it's a shame steam doesn't offer older versions of the games, it's always auto updated, and the client ignores older machines, that's why something like GoG is far better, DRM free, but still their games are normally patched for newer OS/hardware.
in any case, it shows the value of cracking those .exe to remove DRM for games preservation,
another thing is that Hl1 on steam runs with forced anti aliasing and anisotropic filtering, so you have to force it off on the graphics card driver (if you are running a modern card at least), and as I said the soundtrack is terrible (64kbps mp3), or maybe with some command (which I don't know) you could disable AA,