These on join/part messages (from infected users) contain porn sites, which you are suggested to visit.
Such "infections" are caused by javascripts that these sites contain. These javascripts write their own scripts in the mirc directory and load it in mirc's remotes, as well as unload all other remote scripts.
Note that, these javascripts create instances of the remote file in EVERY directory which contains mirc.ini and that it looks in all drives that a user may have in his/her pc.
All some of these remote files do, is just to advertise the website.
Some of them turn the infected user into a bot (thus use him/her to connect to certain IRC servers and flood channels or users) using socket commands.
The filenames of these remote scripts, usually are:
script.ini, script2.ini, server.ini (not servers.ini), versions.ini,
versions.txt.
PS. There has been an update for jscript.dll and I think it was about to fix this exploit. Go to microsoft's site and search for Q814078.
