Generally speaking, I tell people not to install free software on a computer anymore unless it’s licensed under the GNU GPL or another similar open-source license, because open-source software is the only type of software that has any high degree of likelihood of not containing adware or spyware or other malware.

The problem with that advice is that the people who know what it means probably already follow it, and if you follow this Farquhar’s Law (there are many) to the letter, you miss out on gems like Irfanview.