Just yesterday, Microsoft released a 'critical update' for issue 951748, fixing a DNS security hole. It turns out that this fix completely hoses the Zone Alarm software firewall (which I happen to run). In essence, you completely lose your internet connection.
The folks that make the Zone Alarm firewall are aware of the problem. For now, they suggest two workarounds: either uninstall the MS fix or set the firewall security slider to medium (down from high). Hopefully, a true fix will be issued within the next few days.
Update: This problem has even made Slashdot.