I get disconnected every five hours after connecting to the internet, on windows 98, linux and XP - on every computer in my house, even my brother's new alienware system.  I timed it, and sure enough at exactly  5 hours of being connected, it clicked off and I had to re-connect.  I think its the ISP's fault; maybe they disconnect you after a set amount of time??  Perhaps call your ISP and ask them what's up and see what they say.  I called my isp a long time ago about this but they gave some bogus directions to enable something in dial-up networking which didn't help worth a crap.  I might call them again and pester them about it, but I'm going to college soon and I'll no longer have to use dial-up.  Yesss!!
I don't know if this problem lies with the isp, modem, computer settings, or whatever.  Maybe being disconnected on dial-up just a fact of life.  Is there anyone who has a dial-up connection where they never get disconnected? It would be interesting to hear from them.