A patch it released earlier this month had the unfortunate effect of conflicting with third party firewalls and switching off the Interweb.
Microsoft claimed that only a small number of people suffered from the problem, and it will fix the update as soon as it can.
The 21st of May patch can be removed by going to the control panel under the add/remove option, and blitzing it.
Last week we revealed that Microsoft doesn't even recommend its own firewalls. µ
See Also
Microsoft says its own Firewalls have massive holes