I have a windows 2000 pc with a AMD processor and 512 megs of ram... the computer was running fine up until about a month ago. However, when I use it for more than 20 minutes at a time it spontaneously freezes. There is no warning or event id's.. it just completley freezes. I replaced the network card and the ram - formatted and reinstalled the operating system and it still does the same thing? what else could it be?