I re-installed Arch last night as I noticed a while ago I was using 64bit and had been meaning to get around to switching to 32bit.

The install went fine, but when I install xorg and a window manager (I tried dwm and openbox), and start a web browser (I tried midori and luakit), my laptop shuts itself down as if it had overheated or something similar.

At first I thought my laptop was having trouble, but I'm currently booted into a manjaro livecd (I know), and can browse etc. with no issues.

Any ideas on what might be causing this, or what I can post here to help troubleshot?

Thanks.

is this related to a specific url?

Manjaro is fully fledged system that comes with power management already configured. It may be possible that on your installed Arch system power management is not configured properly. I would recommend top check the temperatures.