Microsoft is signaling that it plans to ship an emergency software update on Wednesday to fix a dangerous security hole in its Internet Explorer Web browser that thousands of compromised Web sites have been using to install malicious software. Microsoft says the critical flaw is present in all versions of IE, from IE5 all the way up through IE8 Beta 2. In an unusually frank blog post, the company estimated that about 0.2 percent of Windows users worldwide may have been exposed to Web sites containing exploits that try to attack this vulnerability. While one in every 500 IE users may not sound like a large number, Microsoft said the frequency of attacks is increasing dramatically. “That percentage may seem low, however it still means that a significant number of users have been affected. The trend for now is going upwards: we saw an increase of over 50 percent in