We have Norton Antivirus Corporate Edition running on Microsoft 2000 servers on our network, but we

First, make sure that Norton Client's running on the PC's are getting updated with the latest virus definitions. It is always better to only allow the NA server to perform the live updates of virus definitions and other components from the vendor's support site. On your network, configure clients to receive the updates from the NA server, which will save bandwidth while allowing you to keep a track of latest updates.

The other important factor to consider is the XP patches - the actual reason behind the spread of Sasser on your network is an existing vulnerability in the Windows XP OS for which Microsoft released an update. Use the Windows Auto update feature to download and install security patches and other updates.

Use this link to understand the actual vulnerability To remove the worm, try these links.