Apple has released QuickTime version 7.6.4 for Mac OS X 10.4.11, Mac OS X 10.5.8 (
http://support.apple.com/downloads/QuickTime_7_6_4_for_Mac), Windows 7, Vista and Windows XP SP3 (
http://support.apple.com/downloads/QuickTime_7_6_4_for_Windows).
The update closes four security holes through which an attacker could infect a systems. Viewing a crafted file in H.264, MPEG-4 or FlashPix format by, for example, downloading the file through a movie portal, was sufficient for infection.
The update for Mac OS X 10.4.11 and Mac OS X 10.5.8 is 57MB, while the Windows 7, Vista and Windows XP SP3 version is 31MB in size.
For more on these fixes here is Apple's view on them,
http://support.apple.com/kb/HT3859