The Microsoft security update KB973525, issued on Oct 13 2009, fixes some vulnerabilities in ActiveX running under Internet Explorer.
Quoting from that article: “Known Issues. None”
Unfortunately, a serious side-effect is to make Microsoft’s own Visio Viewer refuse to load Visio files. The viewer, which runs as an Add-on within IE, only produces a little square with a red cross (in Windows XP) or a failed-image icon (in Windows 2000). Both the 2003 and the 2007 versions of Visio Viewer fail.
Until Microsoft issues an official fix, go to Add/Remove Programs and select KB973525 for removal.
Visio Viewer is then much happier: