There's not much to work with here but just last week Norton expired on a friend's PC and locked him out of the internet "just to be safe." The Local Area Connection would have to DHCP an address from the Apple so that's a lot to ask but tell me how you assign addresses.
For the past year I've been connecting to wifi with an old macbook (running OSX 10.4.11), and sharing it with a Desktop PC (running Windows 7) via ethernet cable. Yesterday it suddenly stopped working for no apparent reason. The wifi connection is fine, and the macbook says that it's sharing it, but the PC has no connection.
I ran windows network diagnostics and it said '"Local Area Connection" doesn't have a valid IP configuration'.
I've tried rebooting the modem and using a different ethernet cable to no avail.