I recently got ADSL, and was having a problem with it disconnecting at random. I was lucky if the blasted thing would stay connected for more than two minutes.
I thought it was line noise but it turns out that the problem was my video card(!). I turned off all hardware acceleration and now my connection is solid as a rock. Apparently its a problem with older video cards (mines an old Voodoo 3 3000). You can turn off acceleration under your advanced display settings.
My ADSL modem is a PCI Rockwell based AccessRunner modem.
I discovered the real problem was that my modem was sharing an IRQ with my video card... I moved up from Win98 to Win2000 and thought this had solved the problem, it hadn't really my modem was now sharing an IRQ with the sound-card it disconnects less but still disconnects quite regularly for now I just disable my sound card when I want to go on the net, and re-enable it when I need sound . Not ideal. But no matter how much I move my modem around the IRQs my soundcard seems to follow it.
If you have this problem change the PCI slot that your modem is in, that should work for almost everyone. It's just hapless arseholes like me that it doesn't work for .