Windows Update KB2753842 Will Make Some Fonts Stop Working
This particular update is labeled as KB2753842 (described as security update for OpenType Compact Font Format). If you have not already installed this update, do not install it! In fact, you should tell Windows Update to hide it so you will never be asked to install it.
If you have already installed it (you'll probably know because fonts have stopped working), go into Control Panel and uninstall it. In Windows 7, it is found in the "Programs and Features" control panel on the "Installed Updates" tab. You'll probably see a lot of updates there so it may take a while to find this one. Just remember the name of "KB2753842" and you'll find the exact one to uninstall.
So what is the problem with this update? While it claims to prevent certain rogue (infected) OpenType fonts from working, it will stop all PostScript Type 1 (PFB) and all OpenType fonts containing Type 1 data from working in applications such as CorelDRAW and QuarkXpress. There may be other programs affected as well. Since this update is so new, more data is coming available all the time.
Given the problems caused by this update, I'm going to guess Microsoft will release a different update in the future to address the security problems. Should you see a new update appear that claims to address OpenType security issues, proceed with caution until you know it is safe to install!
12/14/2012 Update: More information on this problem has been provided at More Details on Windows Update KB2753842 Breaking Some Fonts.