Fluxbox is an extremely light graphical user interface that will run quick and smooth on even old computers without much processing power and ram. Many times it's difficult to find a distribution that runs Fluxbox as many don't have Fluxbox editions as it's not quite as popular as KDE, Gnome, or XFCE which is typically the low resource interface of choice. For those wanting Fluxbox there are a couple of options.
Regardless of which distribution you have chosen to use, you can always install Fluxbox onto your current machine. There isn't a limit on how many interfaces that you can run on your computer at any given point in time.
Search your package manager and look for Fluxbox. Those using Debian or Ubuntu will find Fluxbox in the packages ready to go. Install and log out of the machine. There will be an options menu on the log in screen with a selection to switch sessions. This will let you boot up in whatever interfaces you currently have installed.
Depending on your needs, you may have to add on more packages to get Fluxbox to act the way you want it. For example, by default there will be no notification icons that many have grown to rely upon. You can also get some extras to make your workspace and menu functionalities have a bit more options and features if this is something that you want.
If you are having trouble with Fluxbox after installation and getting everything to correctly work together, consider trying a distribution that already has set everything up properly like the Linux Mint Fluxbox edition. It is based on Ubuntu and probably one of the most polished Fluxbox editions there are. It not only looks nice but runs extremely well as it's been configured and set up by people who really know how to use Fluxbox.