If you have already addressed any problem related to weak power supplies, drivers or system pests, I would check to see if the modem has a feature to allow it to reset the PC from a remote location and disable that feature. If so, turn it of. It is also possible that the reset, if XP is your OS, has detected a fatal error. By design, it can reset your system rather than hang it. If this is the case, we may still be looking at a hardware issue. Good luck.