Why not write a web app ? Supermium is modern chromium for xp and will work.

Just stick to vanilla web technologies and it should be fine.

I'm not familiar with GUI development on XP, but could you accomplish this with a script that you run in the terminal? That might be easier than dealing with a GUI that needs to work on both old and modern systems.

You could alias it to a command like getGameKey your_search_query on both to keep the usage the same. You may need a slightly different command for each one (using find+grep on Linux, some Windows command line equivalent on XP)

Step 1. Install Steam Step 2. Open Steam Step 3. Use the menus to import a game, insert Key.

Steam does this.

You could use Lazarus, which is based on Pascal, cross-compilable, and allows you to create the UI by dragging and dropping.

Another alternative is B4J, which allows you to write the code in a dialect of BASIC, but compiles it to Java bytecode.

rust and tauri/dioxus, possibly egui?