Internet Explorer Security Warning: Microsoft Cautions On Zero-Day Browser Exploit

* Company says PCs vulnerable to attack by malicious sites

* Microsoft says free security tool can protect against attacks

* Warning affects hundreds of millions of Internet Explorer users

* Security experts say it may be easier to use another browser

By Jim Finkle

BOSTON, Sept 17 (Reuters) - 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 mitigate 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 software maker advised customers on its website late on Monday 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:

The EMET software must be downloaded, installed and then manually configured to protect computers from the newly discovered threat, according to the posting from Microsoft. The company also advised customers to adjust several Windows security settings to thwart potential attackers, but cautioned that doing so might impact the PC's usability.

Some security experts said it would be too cumbersome for many PC users to implement the measures suggested by Microsoft. Instead they advised Windows users to temporarily switch from Internet Explorer to rival browsers such as Google Inc's Chrome, Mozilla's Firefox or Opera Software ASA's Opera .