Your chipset appears to be supported by the kernel, so that shouldn't be an issue. It could be that (according to the arch wiki) the required driver isn't loaded because of a conflict with ath_pci. Try this as root:
I don't think you'll have to restart the network service, but it couldn't hurt. If your network suddenly comes to life, you simply need to blacklist ath_pci and add ath5k to your modules array in rc.conf. If that doesn't fix your issue, you might be missing wpa_supplicant from the core repo. If that's missing, you'll need to connect with a wired connection (or get it onto a usb stick and copy the .pkg.tar.gz onto your laptop), install it from pacman, and then try again.
Like Firey said, the Wireless setup page
on the Arch wiki is very helpful. Might be worth a good read (although you're only interested in the Ath5k bit when it starts talking about drivers).