Computer freezing irregularly while online

When im online (dialup) for no obvious reason my computer freezes, sometimes 2 or 3 times in an hour and sometimes not at all. I was just wondering if i could get some help in where to start looking for problems and or causes.

-It has hapened using different browsers - Mozilla, firefox, iexplorer
-With and without msn messenger running
-While running freeRAM (i thought it mite be a memory prob)
-On many different sites, but no one sites causes it on each view
Always have norton internet security running (fairly high security settings)

This started hapening after i installed a new hard drive, which i partitioned and installed winxp and sp2, and all programs onto. Its begining to happen more often as time goes on. When i happens i press reset button an windows boots as though nothing has happend (no "your system has recovered from............")

Id appreciate any advice thanks