Reboot system to connect

Hi, here is my situation:
Had to reformat my hard drive and re-install Windows XP home edition. Then updated XP to SP2 and all the other critical updates. I am currently on Bellsouth DSL using a Westtell modem, once I shut down internet explorer or mozilla and after approx 2-3 hours sometimes longer of inactivity I can no longer connect to the internet, I have to reboot the computer and then I can get back on. Now all my lights on the modem are green, including the "internet" light. I called bellsouth and they had me make a change to my connection from "on-demand" to "always on", but it still does it. So I am not for sure if its a connection problem or a windows issue, never had this before I reformated and re-installed.

Thanks

Phil