Mercurial comes with a built-in "hg serve" command that you can use to serve repositories through http. It creates a web server that you can access through any web browser. Unless you need authentication or you have a lot of users you don't need to setup any external web server.
Otherwise setting up Apache + mercurial on windows is not very hard. If you need help please drop me a line.
Thanks for the offer of help, but we're OK with the web server side of things. I was just suggesting that one possible reason for Git's popularity compared to Hg's is that it isn't a walk in the park to set up a common repo for a team with Hg. If you've got someone who's familiar with setting up a web server anyway, you'll be OK, but with Git you don't need to do anything like that at all.
I don't sure I understand what is the problem with setting up a basic mercurial server. If you have TortoiseHg you just open your repository, click on "Repository / Start Web Server" and you are done. If you have bare mercurial just cd to your repository and execute "hg serve".
Perhaps you have some other requirement (e.g. authentication) that I did not take into account?
I suspect hg serve is fine for temporary use, but it's not really designed as stable, long-term solution. As you say, it lacks authentication, which isn't ideal (or allowed at all) in some circumstances. Also, it needs to be started manually, so it needs some sort of supervisor process/start-up script to be set up.
Obviously this isn't some horrific burden, but it's still more demanding than the basic server set-up for some other DVCSes. The original question was about the reasons for the relative popularity of different systems, and if we're talking about people who are making decisions about a DVCS for the first time, they're not experts already and this stuff probably does make a difference.
Otherwise setting up Apache + mercurial on windows is not very hard. If you need help please drop me a line.