I can't get online with AirPort, please help!

I have a new MacBook Pro from work. If I open in Windows XP I can get wireless access via Mozilla no problem and, if I open with OSX (10.5.8) I can get online via Safari (4.0.2) with the ethernet cable attached, but not with AirPort. I have run the diagnostics and it tells me that I am connected to the network, but the page remains blank with the loading bar whizzing round! I'm not very technically minded, can someone please help me?!!