norton restarting my systems

a LAN of 10 systems getting internet through a vsat modem with norton as antivirus. then suddenly systems started restarting even after reformating and installing karpersky. note window xp on sys

any help

Are the restarts like every hour, every day or ...?

Automatic windows updates can cause restarts when scheduled to install. As far as I remember, By default it does that on SP2. Since you've just reformatted, I would expect few restarts after windows updates.