Legit Reviews warns a Windows 7 update that was rolled out on Tuesday can cause systems to enter an endless loop of reboots:
For those users who are still on Windows 7 (a large population) who have recently installed the latest patch or worse yet, let Windows install them automatically may find themselves with the inability to successfully boot and load the OS. The patch, intended to fix a vulnerability, is referenced as security update 2823324 and Microsoft is recommending that anyone who has installed it to uninstall it, provided they can actually boot up to do so. In the meantime, they have removed the update from the list of Windows updates to prevent further customer impact. For those on Windows 8, it looks like you are safe for the moment. Those impacted can follow instructions here to remedy the issue.