I have been connecting to the Internet using Centrino Mobile
Technology both at home where I have a Linksys Wi-Fi Router and at
university.

However it seems that I have inadvertently changed a setting on my
laptop as I cannot now get access at either location, either
wirelessly or via a cable, whereas  other computers on the networks
are able to.

The problem seems to be connected with the fact that I am unable to
move the "Preferred Network" to the "Available Network" field even
when clicking the 'Refresh' button.

When I had a similar problem it was because I messed up Zone Alarm,
the firewall.  When I turned off my firewall, it connected.  if you
have not eliminated firewall, try it. of course, if you have no
firewall, that obviously can't be the problem.

In my case, the firewall asked me for permission of some really
strange file to access the Internet, and I assumed it was a spyware
file, or some update request and told it no.  It was in fact part of
the Internet system, as odd as the name was.

Of course, it might not be this at all. But, thought i'd mention it. 
Do remember to turn on firewall again later.