if i have realized right you can dial the isp number using your modem and connect the isp with no prob, but you can not browse the web.
there can be a lot of possibilities:
1)your g/f has changed the internet options of your browser (things like proxy setting and ...)
2)the browser was damaged (install it again)
3)the isp has some problems(contact your isp)
4)the operating system is damaged (especialy windows 9x)

.......

without having more information about the problem i can not help much!
(the first thing you might want to check is the internet options ,then ping the isp using
command prompt if you got a reply it means that your isp is working .....)
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