Update – December 29, 2012
Microsoft has re-released this update to fix this problem. According to Microsoft:
The original version of security update 2753842 had an issue related to OpenType Font (OTF) rendering in applications such as PowerPoint on affected versions of Windows. This issue was resolved in the version of this security update that was rereleased on December 20, 2012.
Original post – December 19, 2012
Sibelius users on Windows have reported that their Type 1 and OpenType (OTF) fonts have recently stopped displaying in Sibelius 7. The cause of the problem appears to be the recent Windows security update KB2753842 (described as a security update for OpenType Compact Font Format).
For now, the workaround is to uninstall the security update. For more information on how to do that, see this article.
Keep in mind that uninstalling the update will leave you vulnerable to the underlying security issue. You will have to balance that risk against the inability to work with Type 1 fonts in Sibelius 7, for now.
Microsoft is aware of the issue and has stated:
We are aware of issues related to OpenType Font (OTF) rendering in applications such as PowerPoint on affected versions of Windows that occur after this security update is applied. We are currently investigating these issues and will take appropriate action to address the known issues.
Hopefully, the issue will be resolved in due course.