Usenet is a world-wide distributed discussion system consisting of a set of newsgroups (http://www.faqs.org/usenet/).
There is no English-language newsgroup that is exclusively dedicated to human-computer interaction. HCI topics are usually covered in comp.human-factors (FAQ: http://edgarmatias.com/faq/, last updated Nov 5, 1995). There is also a small German-language human factors newsgroup, de.alt.sci.ergonomie.
In the HCI area, mailings lists tend to be of higher quality than newsgroups.
If your Internet service provider does not carry a particular newsgroup, you can access it through an Web-based news reader service, such as: