Network working but no Internet connection on Computer 'B'

We are running 2 laptops both running Vista Home Basic. We set up a Wireless ADHOC connection to enable both computers to access the Internet, via a wireless prepaid dongle(broadband). It all worked! Then suddenly 2 days ago the Internet connection dropped out on the second computer 'B'. Can anyone explain why we now cannot get the Internet connection on the second computer even though the network itself is working fine? We have tried both Ethernet and wireless. We can access the internet on the host computer"A" and we can access folders and files in the two networked computers but the internet connection just is not there on computer'B'. We are by now almost bald and ready to throw both computers through the nearest window!
ColnPam

Did you do anything different on Computer B that you didn't do on Computer A like add any new software or hardware ?

Nothing different on either computer. We had the Network, set up and working, and then suddenly a couple of days ago we got this problem. The only thing we can think of is we did do Windows updates, but can't remember now whether that was before the problem or after! We are sure it is just a setting got changed somewhere but what?! We have tried setting up a new wireless network, and also an Ethernet network and still get the same results. Logically if the network is working then provided we have ICS enabled on computer A it should work.Cheers ColnPam