I mean I can remember about year three gravity got hit by some people that were mad about the fact that basically gravity stole their money and let bots wreck the server, ever since then at least once a year this happens.

The next most probably source is using the same or similar password at other places such as forums. Passwords could be harvested from those sites and then applied to registered accounts having the same or similar names. In fact, one of the sellers of in-game money has an interview on youtube and he said the same thing. Advice from a zeny farmer? Why not?

Scammers are clever and motivated. Even my former guildmaster was scammed out of billions. Recently, a major guildmaster was scammed out of 14 Bzeny. There are plenty of selfish people that are highly motivated to screw you over.

As for free antivirus such as Avast, you can also set exception subdirectories or sets of files to be exempted from extra-sensitive heuristic checking. Thus, visual basic *.bas files in certain folders could be excepted whereas other visual basic files would not.