Can't share network with firewall on, can't reset firewall to default

I suddenly noticed my desktop wasn't on the network any more. On inspection, network discovery and file sharing was disabled. I tried to enable it, but it would not. I found out if I turned off the firewall it worked fine. I tried to reset the firewall to default, but I get "Could not restore the default policy: error 3". How can I reset the firewall to default so I can turn on network...