Sounds like an issue with the nforce drivers (borked your network controller)
try uninstalling your NIC from the device manager and let windows re-detect it. (by re-booting)
If that doesn't work, try UN-installing the nforce and video drivers before reinstalling new ones over the top of them.
Is BOINC the only app with this problem....? ( <---- important question here )
-Sid
(afterthought: Is your motherboard/BIOS one of those with the firewall built in? If so, your nforce update probably reset ti to a configuration requiring attention to allow BOINC to use the connection)