After Effects 7 users, take note: there have been some interesting and unfortunate developments with the recent release of the QuickTime 7.4 update. Apparently, it disables After Effects 7 and will not permit users to complete rendering videos. There have been some arguments about what is causing this error, with some claiming it is related to DRM issues (let's hope not), while others claim it is nothing more than a preferences bug.
Nevertheless, you can not revert back to a previous version of QuickTime once you have updated. In short: After Effects 7 users, beware! Avoid the QT7.4 update at all costs, unless you want to deal with a major headache.