richy99/keithwalton - The CPU does indeed max out at 100%, The board has a socket 478 - it's a Packard Bell Columbia 2 based on the Gigabyte GA-8SIML.
popper - That's a ton of good information - much more than I had expected and it's given me hope for the old box yet

. It's going to take me some time to go through that and experiment with it but I will let you know how I get on - along with more information on the containers, contents and bitrates as a reference point. At the moment the box is running Ubuntu 6.06 with (in most cases) bog standard packages installed - including mplayer - and an oldish version at that.
Thanks again for all of the responses on this.