Hi Andy,
I would suggest you being to look at "Trixbox".
Trixbox is a collection of open source software all mashed up in to one easy install.
It includes the operating system (a version of centos I believe), asterisk itself, and a whole host of other nifty little things that make running an asterisk box a little easier.
Trixbox is the new Asterisk@home by the way, so dont get confused.
I recommend you head over here,
Current Downloads | trixbox , download the ISO you need, burn, install on a spare box, and start playing!
Matt
Any other questions just fire them my way!