Install stable fvwm with the following command:
# pacman -S fvwm
Unstable release is of version 2.5.28, which has been released quite recently (20-Sep-2009). Although it is called unstable, but it has no stability issues at all. To install it:
# pacman -S fvwm-devel
or, if you have archlinuxfr (see Unofficial user repositories) added to your pacman.conf:
# pacman -S fvwm-patched
One should not mix fvwm window manager and the project FVWM-Crystal, which can be also found in Arch repositories.