You have installed a very old version of Ubuntu, released in April 2008.
Where there are problems, such as yours in refusing to shut down, they are
generally fixed in the next revision. Unless there is some very unusual
reason not to use the current release (12.04 ) you should as it will give
you better results. It is a long term release and supported for 5 years.
The price is the same: free to download, burn to CD and use.