Gor3

It's a false positive. Don't worry, it's completely safe. It has to do with the way the client scans some files that are on your pso folder to make the game work, making your antivirus assuming its a virus/trojan/etc based on its behaviour (i'm not an expert on the area, but other people can explain you in a more detailed and accurate way, but the idea is that).

It happened to me some weeks ago. My windows defender started to getting triggered by it for some reason. My bet is that it has to do with a patch or an update of the game in a few weeks or months ago. Just add an exception to your PSO folder and you'll be fine.

As i said, don't worry with it. Completely harmless. If it had virus on it, nobody would be playing PSO in ultima these days lol