Aiming well below specs - consider 90s abandonware. CDogs, Eradicator, Take No Prisoners. Or really lean into the AOL-era experience and download PC Gamer demo discs from Archive.org. A ton of DOS and Windows 95 titles had shareware that was up to a third of the entire game.

2000-2010 was a weird era for anything free… because it was all Flash. But fortunately all of those janky vector-graphics titles, made for nothing but fun and internet fame, are available via Flashpoint Archive.

Some exceptions that are still toward 2000: N (later N+ and N++), Cave Story (the first version), Nexuiz, OpenArena.

That machine will also run any Game Boy Advance or PS1 game without breaking a sweat. Good N64 emulators might spin the fans.