QuickTime 7.7 has been released for Mac OS X Leopard and Windows 7, Vista, and XP. The update addresses several security holes in this older version of QuickTime and is a recommended update for systems running QuickTime 7.6.9 or earlier.
Mac users who are running Mac OS X Snow Leopard or Lion need not bother with this update, as QuickTime X is the default version of QuickTime Player on those systems and is not under the umbrella of this security update.
More information on QuickTime 7.7 is available on Apple's site. The update is about 70 MB and should show up if you run Software Update.