Sasser Worm Spreads, What You Need to Know
The worm has positioned itself as one of the quickest-spreading and most virulent ones around, suggesting that the number of incidents will soar at the beginning of the week, according to Luis Corrons, director of security vendor Panda Software SL's PandaLabs unit.
Computers infected with the worm boot up normally but then hang up or shut down when users attempt to do any work. The new worm exploits the LSASS (Local Security Authority Subsystem Service) remotely exploitable buffer overrun vulnerability first reported by Microsoft on 13 April in Microsoft Security Bulletin MS04-011.
The worm does not damage files and is relatively easy to remove, although concerns have been raised that information stored on an infected computer could be compromised.
To protect your computer against Sasser and its variants, do the following:
Step 1: Enable a Firewall
Before you take other steps, make sure you have a firewall activated to help protect your computer against infection. If you have a hardware firewall in place for your home or workplace connection, or if you use the firewall included with Microsoft® Windows® XP, the Sasser worm is most likely blocked. If your computer has been infected, activating firewall software will help limit the effects of the worm on your computer. For comprehensive guidance to installing and enabling a firewall, see the Microsoft Protect Your PC site. 
Step 2: Install the Required Update
To help protect your computer against the Sasser worm and its variants, you must first download and install security update 835732, which was released with Microsoft Security Bulletin MS04-011. You can find update 835732 on the Windows Update Web site  listed in the Critical Updates and Service Packs section. You can also download and install this update manually from the Microsoft.com Download Center. To find the download for your operating system, refer to Technical Security Bulletin MS04-011 .
Note: If you installed the updates for MS04-011 manually or through Automatic Updates before Friday, April 30, then you are already protected against this issue.
Step 3: Automatically Check For and Remove Sasser.A and Sasser.B
You can use this tool to search your hard disk for and try to remove Sasser.A and Sasser.B. To do so, click Check My PC for Infection.
Important To use this tool, you must be running Windows XP or Windows 2000, and you must have already installed the update released with Microsoft Security Bulletin MS04-011.