Pro Web Pages Have Stopped Loading

I have a friend who lives in another part of the country so I cannot actually get to her computer. Over the past two weeks we have sent e-mails back and forth trying to find a solution to the problems she is having.
When she first starts up her computer and connects to the Internet (she is using WinXP Pro and a USB modem connected to braodband) she can surf webpages OK and get to her Yahoo webmail account.However, after about 5 to 10 mintues, if she tried to get to a webpage again, including Yahoo, she gets a "page cannot be displayed" message and it is impossible for her to load any webpages at all.The Internet connection is still connected, she can send and receive e-mails using OUtlook Express, but she cannot get web pages to load anymore.We have cleared the caches, done antivral and spyware scans using several different programs (her system is clean) and have tried using both Internet Explorer and Firefox.

Both browsers "freeze" after a time and she gets the error messages saying pages cannot be displayed.Sometimes when she reboots her PC and connects to the Internet she can get web pages to load again but after a time surfing "freezes up" and webpages no longer load. BUT, as I said, the Internet connection is obviously fine because it does not affect the sending or receiving of e-mails using Outlook Express.