If you use a Windows computer connected to a network, a newly discovered bug makes it possible for a bad guy to wreak havoc on the computer without your doing anything. The most vulnerable versions of Windows are XP, 2000 and Server 2003. Vista and Server 2008 are also vulnerable, but not as badly. Microsoft considers the bug important enough to issue the patch immediately rather than waiting for their normal once-a-month patch Tuesday.
Susan Bradley, writing for the Windows Secrets newsletter recommends immediately installing the just-issued patch. Then she offers some unusual advice, suggesting people first restart their computers "to verify that your machine is bootable." Can't hurt. Then she says to install the patch and reboot again. Her article also includes direct links to the patch for each version of Windows. If, for some reason, you can't run Windows/Microsoft Update you can manually download the patch and install it.
A standard of Defensive Computing is that the less software installed and running the better. This particular bug is with a part of Windows known as the Server service. If you are not sharing files and/or printers on a local area network, then you don't need to have the server service running, bug or no bug.
Making a Windows service not run all the time is called disabling and/or stopping. Stopping refers to the instance of the service currently running. Disabling means preventing it from ever starting again. Microsoft describes how to both stop and disable the Server service in Security Bulletin MS08-067. They also suggest doing the same to the Computer Browser service.
Anyone not sharing files and/or printers on a network should also turn off File and Printer Sharing for Microsoft Networks (the Windows XP name) on all network definitions. For example, on a laptop with both wired Ethernet networking and wireless Wi-Fi networking, File and Printer Sharing should be turned off in both network definitions.
If the Server and Computer Browser services are disabled, then some people might consider the last point (and the next) overkill. I think they are a good idea because it means two mistakes would have to be made to enable file and printer sharing as opposed to only one mistake.
For still more safety, look into how your firewall is configured to ensure that it does not allow incoming traffic on TCP port 139 or 445. Again, this is for someone not sharing files and printers. Firewall configuration varies widely, but if you are using the Windows firewall in XP, the exception for this is called "File and Printer sharing."
Firewalls are the first line of defense against this type of problem. With that in mind, you may want to review the series of postings I did recently on adding a second router to a LAN to provide additional firewall protection to your most important computers. See A second router protects adults from kids.