Microsoft has released 12 security updates, nine of which it deemed critical.
One Windows gap can let hackers get into computers that haven't entered fraudulent Web sites or downloaded infected files. That hole affects Windows 2000, Windows XP and Windows Server 2003 programs, said Oliver Friedrichs, director of Symantec Security Response.
"You can become infected simply by being connected to the Internet and leaving your computer on, if you're not protected by firewall or antivirus software," Friedrichs said. Symantec makes Norton security software.