I am running Xubuntu 9.04, with LXDE on an old Acer 526iTX. After fiddling around trying to connect it to a Vista machine using a crossover cable, I must have broken the internet (well not the whole internet obviously, just my connection). nm-applet will show the connection as working, but no loading takes place (eg. firefox shows 'connecting to google.com.au..' but nothing happens). Same behaviour whether browsing, pinging or updating by CLI, and over any type of connection (mobile broadband, wifi or ethernet cable). This has persisted over many days, through restarts and deleting and re-creating the connection in nm-applet. I can also rule out a hardware problem as the connections work fine using a live CD.

It's no big deal if I have to reinstall (I probably will anyway), but I was wandering if anyone would know the probable cause of this behaviour.

why are you using a crossover cable to a vista machine? do you have a router or switch?
is this a DSL or cable connection.
or are you running internet connection sharing with vista?

I was playing around, trying to get it to work. I have a switch now, but I haven't tried setting it up or continuing with my attempts at networking since the internet stopped working on my laptop.
Neither. I don't have a phone line or cable at home so I use a usb mobile broadband modem. I have tried a DSL connection through a switch somewhere else, and wifi but the behaviour is identical no matter what method of connecting.
No, I switch the usb modem around to whatever computer I want to connect to the internet with.

The problem is confined to the existing installation of the laptop (a live CD will work fine and no problems on any other computer).

I never figured out what was going wrong with the internet connection, so I reinstalled Ubuntu today.