Some time ago I bought a physical copy of Rat Attack for PC, because I had fond memories of playing it on a Playstation Demo Disk.
Unfortunately, while the game itself technically works, after I get past the first “chapter” (“House”), I get a message saying “You need to download the next part of this game to continue. Visit the Freeloader web site? Press OK or Cancel to exit the game”.
Pressing OK does nothing, while Cancel does indeed exit the game. Looking at the game files it seems that all of the files are there (I can see the files for the other levels and they seem to have the same size as the one that loads correctly).
Freeloader no longer seems to exist and even searching sites like myabandonware turns up empty for this game. Am I screwed and this game is lost media now, or is there anything I can do to get it to run past the first couple of levels?
While I’m not familiar with that game specifically, it sounds like your hitting some sort of DRM issue where the retail game is stuck running in demo mode. My first bit of advice would be to check gamecopyworld to see if there’s a crack and install it if there is. There may also be a cracked version on archive.org.
If that doesn’t work, then it’s likely the game is using some deprecated Windows feature, or doing something in a hacky or unintended way that’s been patched out in newer releases. In that case, running in compatibility mode might fix it, but the chances are slim. A VM, like @wolfshadowheart suggested, would have a much higher chance of success.
If all of the above fails, the last hail Mary you can try before tracking down an era appropriate computer, is running the game in Linux under WINE. Ironically, Linux has better compatibility with a lot of old PC games then Windows these days, but it takes a bit of fiddling to get working.
Edit: Another resource to check is the PC Gaming Wiki. They have dedicated pages for most PC games, with a list of common issues and detailed instructions for fixing them.