Late last week Apple released a security update for QuickTime for Windows. The update brings QuickTime to version 7.7.8.
To update Apple QuickTime for Windows, open QuickTime (by searching for it using the Start menu). From the menu bar at the top of the QuickTime window choose Help->Update Existing Software
Alternatively use Apple Software Update (usually installed with Apple iTunes). Upon opening Apple Software Update it will check for updates for you and display any applicable updates for QuickTime.
As always, I recommend backing up the data on any device for which you are installing updates in order to prevent data loss in the rare event that any update causes unexpected issues.