A Reddit user has reported (https://www.reddit.com/r/simutrans/comments/1btdtcx/problemas/) that the download-paksets.exe was flagged as containing malware by their virus checker.
I am aware that AV programs often produce false positives. And I'm not sure whether this executable is specific to the Steam version or part of Standard itself.
But I am passing the message along in the hope of drawing it to the attention of someone who knows.
There is nothing what we can do. This is created by NSIS, which is also misused by malware. The NSIS installer is highly optimised, self-extracting code and contains download functions to download code. That is enough to flag some scanners from time to time.