According to the update description the patch fixes a stack buffer overflow which existed in the application's error logging system where viewing a maliciously crafted QuickTime movie could have potentially led to an "unexpeted" application termination or arbitrary code execution. In other words, if you've recently watched a video using QuickTime Player coming from any "questionable" source, you're at risk of being affected.
The update comes in the form of QuickTime 7.6.7 and is immediately available for Windows 7 users, Windows Vista users and Windows XP SP3 users. The update can be downloaded here.
