QuickTime 7.0.4 is a critical security update

[Rasheed]

Versions of Apple QuickTime prior to 7.0.4 had a security risk. By playing malicious files, hackers could gain access to your computer (both Windows and Mac OS X) and do harmful things.

So, if you haven't done so already, update QuickTime to version 7.0.4.

See also: Apple's QT page.