Firefox and MSIE problem Vista Acer Laptop

Acer laptop with MS Vista about for months old. I installed Firefox and Thunderbird on two almost identical machines for a colleague. All worked perfectly until about a week ago, one machine only although Thunderbirds and Outlook Express both send and receive emails, and MS, AVG and SuperAntispyware all update perfectly, Firefox and MSIE give the appearance of going on line then just sit doing nothing with a blank screen. MSIE after a while displays the message that it was unable to connect to whatever website was chosen. Both machines have been tried at two different locations using the same service provider and everything else works fine. The machine concerned is not often used and everything was ok at last shut down. Can anyone give me any advice, I have gone through all the things I can think of and checked all settings against the working machine.
TIA
Mart