I have had exactly this problem today. Brand new computer build with Win XP pro. Fresh WinXP pro install with chipset drivers and all XP updates. Absolutely no other software was installed or uninstalled. I next installed Avast 5.0.677, and I lost internet connectivity but still had network connection to home computers. Uninstalled Avast and internet back. Reinstalled Avast and turning on and off web shield turns on and off internet access.