Q: I have a nice little media PC waiting to be put to use, all its missing is a hard drive. I wondered while I'm waiting to get one is there a Linux distro that purely boots from CD/DVD so I can get it folding.

A: Most Linux distros come in a LiveCD (or DVD) form nowadays, using something like Suse or Ubuntu would be good.
Alternatively, something like Knoppix, Slax or DSL could be used. Knoppix is purely a LiveCD, while Slax or DSL could be installed onto an old USB flash drive.