I few days ago I suddenly started getting win32/AgentBypass.gen!K warnings from Windows Defender logging in on Windows XP Professional after booting the system. It seemes to be tied to fumoei.exe and after deinstalling Free Download Manager (and also deleting any leftover Free Downloader Manager installation directories and files after the deinstallation) the warning from Windows Defender has gone away.
Seems like a Windows Defender definition update triggered this warning since I now can't find any problems on my PC after scanning it with various tools.