Can anyone recommend a website that sells DRM-Free Board Games for Windows?

Sadly, the vast majority of board game ports are only released DRM-full-ly. Asmodee (the board game equivalent to EA) scores more licenses than they should and their products tend to be Steam-exclusive on PC (and mobile ports are Store only).

Humble -- way back! -- had a few ports DRM-free, but I don't think they're still available. There is one on GOG (Witcher one). A few more games are board-game-like on GOG, but aren't precisely that.

Could you go into a little more what you're looking for?

There are some open-source or otherwise free tools for board gaming online (VASSAL being one of them). Many popular games get fan-made websites for Internet play [though rarely open-source; not technically DRM-free, but no cost but time involved). Sadly the paid, DRMed, piracy-inducing tool on Steam, "Tabletop Simulator", has DECIMATED both of the prior. :(

I wasn't looking for anything in particular. I just wanted to see what is available.

Antihero, Tharsis and the Witcher adventure game are available on GOG.

There are some other games that have some sort of boardgame mechanics, like Sygma Theory, Invisible Inc, For The King, Guild of Dungeoneering, etc. but not many true boardgames around here, sadly...

Solium Infernum is worth your coin, if you enjoy ruling Hell.