XP PRO HANG-UP ERRORS

I recently upgraded from 2000 Pro to XP Pro. I did a fresh install (not an upgrade) and it seems to be running fine expect that every session which is more than 30 minutes long ends in windows hang up. The screen feezes and there is no other alternative but to cold boot the system. Even if i do a reboot (hardware quick reboot) the system would not start but hang up during the start up.