With the recent release of Release 261, Avast has started flagging HDoujin Downloader as malicious. This happens every once in a while when the program changes enough that it’s no longer picked up by the whitelist (this is a recurring problem with Avast in particular).
The last update has been submitted for whitelisting. As the developer, I can confirm that this is a false-positive, but if you’d rather wait until it’s cleared, usually they whitelist it within 3 days. Otherwise, you may add the program folder to your exceptions list to prevent it from being deleted.
Overenthusiastic “anti-virus” scanners are the bane of small-time developers.
Update: It has now been whitelisted in Avast’s latest definition update!