|What kind of Modem are you using DSL or Cable?|
When you down graded did you get your XP from a bittorrent like Limewire or something?
Most DSL modems will reboot if your computer is uploading too much traffic to prevent Internet Attacks. If you downloaded an ISO for Windows XP then chances are you got one that has a Trojan in it that is either spamming through your internet connection or worse. If this is a clean version then you may be hosting a game server or file sharing server.