VirtualBox: modprobe can't find vboxguest, vboxsf, vboxvideo

I had the exact same problem when i was trying to install arch on virtualbox earlier today. The solution is to run depmod

$ depmod 3.14.4-1-ARCH

After running modprobe again, it should work.

You can use uname -r to find your kernel version string.

Source

I found this thread that sounds like your issue. Titled: Virtualbox guest module doesn't load.

excerpt

Do you have the official virtualbox guest additions installed? You shouldn't do that. If so uninstall them and then install the package

   $ sudo pacman -S virtualbox-archlinux-additions
   $ sudo pacman -Qs virtualbox

should give:

   local/virtualbox-archlinux-additions 4.1.8-2
   local/virtualbox-archlinux-modules 4.1.8-3

Source: Wiki: ArchLinux Wiki - VirtualBox

UPDATE #1

Per a comment left by @DavidGrayson the package names may have changed. So try these instead if the above do not work:

$ sudo pacman -S virtualbox-guest-modules-arch virtualbox-guest-utils