Since they require a referer:
wget --referer=http://launcher.nirsoft.net/downloads/ -m -np http://download.nirsoft.net/nirsoft_package_1.20.10.zip
wget --referer=http://launcher.nirsoft.net/downloads/ -m -np http://download2.nirsoft.net/nirsoft_package_enc_1.20.10.zip
The latter has password nirsoft9876$
The filenames change over time (the 2016 archive of http://launcher.nirsoft.net/downloads/ shows http://download.nirsoft.net/nirsoft_package_1.20.5.zip and http://download2.nirsoft.net/nirsoft_package_enc_1.20.5.zip
Need to check out if I can automate this, as they seem to keep a SysInternals link http://download.nirsoft.net/sysinternals4.nlp
–jeroen