The worm has been designed to mount a “denial-of-service" assault in which a flood of traffic crashes a website. The worm called "W32.Blaster" will prompt PCs to attack Microsoft's www.windowsupdate.com site.
The shutdown was one of several measures taken to protect the site, which provides software and security updates, a Microsoft spokesman said.
The denial-of-service attacks may still slow Internet traffic or networks of infected companies as affected computers clog networks with the data traffic they direct at Microsoft's site.
According to a Microsoft statement, the Blaster worm can potentially impact customers in two ways.
"First, it may cause infected PCs to repeatedly crash.Second, it could potentially use infected machines to try to launch a Denial of Service attack on Microsoft's Windowsupdate.com on August 16," it said.
"First, it may cause infected PCs to repeatedly crash. Second, it could potentially use infected machines to try to launch a Denial of Service attack on Microsoft's Windowsupdate.com on August 16."
This anticipated attack might cause the Web site to run slowly or be inaccessible to customers.
Microsoft is currently taking aggressive steps to try to ensure that Windows Update remains fully available.
In the event of any disruption, customers can continue to access and download the patch by going directly to http://www.microsoft.com/security/, the statement said.
The worm uses a flaw in the Windows NT 4, 2000 and 2003 server programs and Windows XP personal-computer operating system to replicate and spread.
Blaster does no damage to computers, though it does make them shut down repeatedly or run slowly.
Blaster is a worm, a computer virus that spreads by copying itself to other disk drives, systems or networks.
Unlike a virus, it doesn't require a programme or file to latch on to in order to disseminate.
The Blaster worm spreads by scanning the Internet, looking for computers that haven't installed a software update from Microsoft that patches the flaw in its operating systems.