The Worm is spread through the extremely broken RPC service run on windows. RPC is short for Remote Procedure Call. Pretty much allowing a remote host to run commands native on the system. Microsoft usually uses it to push updates or to poll systems if they are a valid copy of windows.
It normally runs as a listener on port 135 or 445 on a host. Very rarely will you ever see it with an established link.
The problem with the service is that it has been patched about 3400000 times in the past, and yet there are still vulnerabilities for it. One would think that Microsoft would come out with a better way to push updates and check for valid software. But they don't. And don't think that you can disable the service and stop it from going on. It is loaded as a kernel level process on a windows system, if you were to disable it, one would not be able to boot to windows (in theory).
Ensuring that you keep a good security policy is of utmost priority. Backing up important files to external media, ensuring windows is properly updated, and that you have a good and up-to-date Anti-virus application installed is a great way to protect oneself. Also having administrative accounts password protected is also another important security measure.
There are a lot of unnecessary services running on a windows machine, but for someone who doesn't have proper knowledge could seriously muck up their system by disabling required processes. Windows has, and probably will always be vulnerable; which is frustrating for security professionals (like I'm training to become) due to it being the most prolific operating system, "necessity" for backwards compatibility (B.C. all the way back to windows 3.1? WHYYYYY!) and the open nature of the system does not help at all.
God I hope Microsoft pulls their head out of their ass and come up with a fucking decent operating system, or fucking fall of the face of the earth and let real operating systems take the forefront (read: Unix Based)