I want to buy some old games and hack them to run in widescreen, but unfortunately some of the games require a cracked exe in order to do that. Is it safe to asume that steam wont recognize the game if I do that? I know it may still work, but I like steam to track my in-game time, and I think it would be a bummer if it detected it as a non steam game. Games that can be modified by cfg files work well with steam in my experience ( like Return to Castle Wolfenstein), but if I want to buy Cold fear I will have to download a new exe for widescreen.

I believe I've modded executables of some of my Steam games but I can't say for sure. The Steam version of Cold Fear should be DRM-free anyway, so the risk isn't that high. Just make sure not to verify the game files afterwards.

Edit: Did a quick check on the Steam community hub and some people seem to be straight up hex editing the Cold Fear .exe for custom resolutions, so you should be fine on that front.

Sometime it will run fine but i have had multiple instances of altered exe files being replaced by Steam with the originals when Steam realises its not the thing it should be. Its not going to break your game or anything but its a constant annoyance, you can tell Steam not to scan that game but you say goodbye to Steam achievements and it will show up as a non Steam game.

if im not mistaken steam has some sort of weird exe encryption thing, it's its own drm, but I guess it's only used on games with steamworks, not sure.

And in those cases it would of course not work because it's a system designed to prevent executable modification.