[link|http://www.tipic.com/|http://www.tipic.com/]
I have no idea how well it works; this is just Googlism.
Cheap as chips, too; $500 for 50 users.
It's more expensive than running jabberd, but you'll run up $500 in time spent quicker than you can say knife if you try and get THAT going on Windows.
On Linux, of course, it's about 20 minutes from zero to hero, and that assumes you've never done it before.