If you only need the functionality of one component, you can just install that component; however, all of the components rely on common code in the Horde package itself. Whether you install one component or all of them, you need to install Horde. So, if you just want to offer Web-based email (by far, the most common use of Horde applications thus far) you'd need to install Horde and IMP.
Horde, and all of Horde's components, are free, having been released under the GNU Public License. Those of you familiar with open-source software can stop reading now. For those of you who are used to paying for software, no, we're not kidding, it's really free. And not only is it no-cost, but it's yours to modify and redistribute, too; the only restriction is that you can't turn around and make it not free. So, you can download Horde and components and install them on as many computers as you want, and let as many users as you want use it, without having to pay a penny to anyone, and you can make changes to the way it looks or operates, either for local use or for redistribution.
The Horde framework itself is even free-er, being released under the Lesser GNU Public License. This basically means that you can use the Horde framework in proprietary programs; if you are considering doing this, please read the license, and, ideally, let us know.
Horde is developed on Unix, with the Apache web server, so it should certainly work on any Unix that you can build Apache and PHP on. Being written in PHP means that it should work anywhere that PHP works, though, so if you're not on Unix, please give it a try and let us know how things turn out!
Horde and all the Horde Components are known to work under Windows NT/2K/XP with Apache and PHP4 with acceptable performance. IMP and Turba, at least, are also believed to work on Windows NT/2K with IIS and PHP4.
The best place to turn after reading the documentation is to the appropriate mailing list for the component you're trying to install. Make sure you describe the problem clearly, including all error messages you might have received. Include the version of all the software you're using, including PHP, your web server, your database, any mail or LDAP servers you might be using, and, if the errors are occurring in use, what browser and OS are being used to access it.
Please don't contact the developers or the FAQ maintainer directly. They usually don't have time to deal with individual installations of Horde and its components, being too busy developing and FAQ-maintaining. (Besides, they're probably on the mailing list anyhow.)
Horde users are encouraged to join the announce list and the list specific to their module; those interested in getting their hands a little dirty might also wish to join the horde and dev lists. Those interested in knowing everything that's going on with Horde development might wish to join the machine-generated cvs and bugs list, which report changes in the CVS repository and in the bugs database.
Before you do anything, take a moment to read this article on bug reporting, and keep its comments in mind when reporting Horde bugs.
Having read that, there are two places where a bug can be reported. If you're on the mailing list for the component you think you've found a bug in, send a message to the mailing list, and be sure to keep an eye out for replies. You can also enter the bug into the Horde bug database.
Be sure to describe exactly how to reproduce the bug (if you know!), and include all error messages that appeared. Also, specify the versions of Horde, the Horde component exhibiting the bug, and all supporting applications (web server, PHP, database, mail server, etc.). Of course, developers love when bugs come with patches attached; but if you can't write PHP, or can't track down the bug, make sure you let us know about it!