You likely have Flash installed to play clips, but need to install codecs to
play DVDs. Install the xubuntu-restricted-extras and while you are at it
install VLC for playing DVDs. You can use Software Centre, Synaptic or the
terminal.

The terminal commands are: sudo apt-get install xubuntu-restricted-extras

and

sudo apt-get install vlc

i installed those and also the recommendations here:
https://help.ubuntu.com/community/RestrictedFormats/PlayingDVDs

But oh dear still don't seem to get the DVD to mount. I can't remember
what else to do. I've used Ubuntu 8.04 up to now and decided to switch
to xubuntu earlier this year but due to other things only got round to
it more recently.

In Ubuntu DVDs do not mount unless you put a disk in. Once you do then you
click on the usb icon in the system tray or notification area. Then it will
give you a choice for that disk when you click on it in the drop down. You
do NOT get a Windows style popup or associated application going to work
automatically. Conversely you can open VLC or similar application and go to
File or Media, Open disc. Then navigate to the DVD drive which is usually at
/dev/sr0 or sr1 depending on which drive if you have two.

Commercial DVDs may require libdvdcss2 which can obtained from the Medibuntu
repositories. It is not available from Ubuntu. However, it has been my
experience that the ones provided by Ubuntu are adequate to do what you
want. You can easily add Medibuntu to your sources. It is even recommended
by Ubuntu and has some packages that are very useful such as GoogleEarth.
See: https://help.ubuntu.com/community/Medibuntu

This resource may be helpful to you:
https://help.ubuntu.com/community/RestrictedFormats/

In Ubuntu, this depends on what settings you have chosen in Nautilus. In the
file manager, choose Edit/Preferences, and select the media tab. One check-box
is "Browse media when inserted," another is "Never prompt or start programs on
media insertion.

I *think* the Ubuntu 10.10 defaults are to browse and prompt.

Xubuntu? I have no idea.