A couple of weeks ago, I lost internet access for about 30 mins due to a Denial of Service (DoS) attack.
Brute-force attacks from individuals are difficult, as my downlink speed is much faster than most people's uplink speed. To mount an effective brute-force attack requires multiple computers infected with a trojan. This is known as a Distributed Denial of Service (DDoS) attack and requires a lot of computer knowledge.
A much simpler approach is the SYN flood attack. I believe that I was subjected to one of these, as on rebooting, I temporarily had internet access but lost it after a few seconds when all of the available connections on my pooter became "half-open".
To defeat such an attack is simple. All that you have to do is harden the TCP/IP stack. See also http://msdn.microsoft.com/en-us/library/aa302363.aspx and http://www.pctools.com/guides/registry/detail/1237.