Microsoft is investigating <A HREF="http://news.zdnet.com/2100-1009_22-6052396.html?part=rss&tag=feed&subj=zdnn">a security flaw</A> that could let an attacker gain control over a vulnerable Windows computer, the company said Tuesday. The flaw was reported to the company earlier this month by Jeffrey van der Stad, a 25-year-old Dutch programmer. The problem is related to the way the browser processes so-called HTA files, Microsoft said in an emailed statement. HTA files are associated with Web applications.