Some usb dsl/cable modems may get disconnected after a little while. A possible reason that could cause this problem is that in windows xp there is an option that allows the computer to turn off a usb device to save power. To disable this option:

1) click start, highlight my computer, right click and select properties and a window will pop up with the system properties
2) click on the hardware tab and press the device manager button
3) expand the universal serial bus controllers
4) Double click on the USB Root Hub.
5)Select the Power Management tab and UNTICK the "allow the computer to turn off the device to save power"

If you have more than one USB Root Hub, then you should apply the steps to each one of them
That's it, you won't need to restart the computer!