need help on screen and net freeze

i have an xp athlon 1800+ chip and 256 ram on a m810 agp board whilst on the internet i seem to suffer on occasion screen freeze / mouse freeze and my computer reboots from the start is there a way to beat this and is there a reason for why it is doing this as i am rather frustrated at several reboots before the system will stay stable on the net.???? Please help…