Intel Proset, Drivers and the Internet

This has happened more than once. I have Windows XP Opsys. Today the Internet was working just fine. I walk away for several hours and come back, no Internet. I do all the bells and whistles, switching stuff, turning all of etc only to find it still didn't work. SO, I contacted the ISP who checked their lines and said it is not our problem but your machine, specifically the network card. You have to download your driver for the Ethernet connection. Well, obviously is was downloaded earlier....what happened and why did I have to reinstall it?