Just wanted to post a heads-up. There is a conflict between Microsoft's update KB951748 and ZoneAlarm. Something about that update will cause ZoneAlarm to block your browsers from internet access and ultimately shut your connection down. ZoneAlarm is recommending that you uninstall this update as a fix for right now. Turning your ZoneAlarm setting down to "medium" will also fix this problem, but obviously will reduce the level of security provided by the program.
Have you changed any hardware recently? As Mr Boffo says it could be loose IDE cable or it may have failed. Some BIOS will have a section where you can clear warnings, hit DEL to enter setup and see if you can find it.