http problem

Hi,

I'm using a macbook pro, 3 years old running mac os x. I'm using airport to connect to our router. I know we have internet because skype works. However I can't open websites - this is true regardless of browser (i.e. I've tried with both safari and firefox). Interestingly, the browsers can access ftp and https addresses, so the problem seems to be a specific problem with http. Any ideas how i could resolve this would be most welcome.

Thanks very much
Sim