The giant software maker Microsoft Corp warned a newly discovered bug in its Internet Explorer web browser makes PCs vulnerable to attack by hackers and urged customers to download a piece of security software to reduce the risk of infection. The security flaw affects hundreds of millions of Internet Explorer browser users. Microsoft said attackers can exploit the bug to infect the PC of somebody who visits a malicious website and then take control of the victim's computer. The company advised customers on its website to install the security software as an interim measure, buying it time to fix the bug and release a new, more secure version of Internet Explorer. The company did not say how long that will take, but several security researchers said they expect the update within a week. The free security tool, which is known as the Enhanced Mitigation Experience Toolkit, or EMET, is available through an advisory on Microsoft's website: http://blogs.technet.com/b/msrc/ .