It sounds as if your ISP is set to disconnect idles connections after a period of time elapses. I have used an application in the past when I was on dial-up called Connection Keeper, available at
http://gammadyne.com/conkeep.htm for Windows. It sends out pings every so often to simulate Internet activity in order to keep your connection alive, thus preventing your ISP from dropping your connection. I do not know of any such programs off-hand for Linux, but since you are on a dual-boot system you should be able to utilize this on your Windows XP partition.