For the second time in a week, Microsoft is warning that criminals are exploiting a previously unknown security hole in its software to break into Windows computers. The company has released a stopgap fix to help protect users until an official software update is available. The problem stems from yet another insecure ActiveX component, this time one made to manage Excel spreadsheets between Internet Explorer and various Microsoft Office products. In an advisory released today, Microsoft said it is aware of attacks exploiting this vulnerability, which is the sort that could give criminals complete control over a vulnerable Windows PC merely by tricking users into visiting a booby-trapped Web site with IE (yes, this means if you use Windows but consistently use a non-IE browser to surf the Web and open e-mail links, then you have little to worry about from this flaw). According to Microsoft, your system is vulnerable