looking at bkav’s HW32.Packed detection, it might indicate, that it is triggered by the nsis compressor setting
SetCompressor /SOLID lzma in nppSetup.nsi, as the result is an .exe with compressed resources.
just like .exe files, that are compressed with UPX.
they often (as in very, very, very often) produce heuristic virus alerts.
and now, with the missing code signing certificate, it might be possible, that the engines do not whitelist the notepad++ installer any more, if they have whitelisted it before.