I my experience the most common torrent client ( installed by default ) seems to be transmission
I honestly don't remember if it was installed by default in Ubuntu 9.10
Simplest solution, use terminal
if that fails, it will probably tell you how to install it, but here it is anyway
sudo apt-get install transmission-gtk
Once installed, it might be easier to simply re-download the torrent file
Hopefully your browser will pop up a prompt "Open with .. save as"
Open with transmission
Alternatively, download direct from http or ftp.
But it is much more polite to use the torrent, as this helps keep down the cost