Apple has released a new version, 7.7.1, of its QuickTime media player for Windows 7, Vista, and XP (with at least Service Pack 2). The player is built into Mac OS X. This release incorporates fixes for twelve identified security vulnerabilities, many of them serious. More details on the fixes are available in this support article.
I recommend that you update your system to the new version as soon as you conveniently can. You can download the new version here.