I had a problem where IE could connect to the Internet, but all other apps (Firefox, iTunes store, AVG virus update) could not.
Looks like after uninstalling Symantec trial AV (came with PC) using Add/Remove programs/Norton uninstaller traces were left behind casuing the problem. There is an uninstaller on the Symantec website that seems to do a better job. After running this and rebooting all is OK.
Hope this helps.