Have you updated to 4.2.1 yet? Such an issue was solved in the update.
Yes - all up to date.
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I should have read the topic title more carefully. Sorry.
All I can think of is some basic housekeeping duties. Clean the media cache database, clean out your User and Windows temp folders (CCleaner is a great free utility for that) and then restart your computer.
If that still doesn't work, then you can try trashing your preferences:
Also, try re-installing Quicktime, making sure you have the latest.
That was it - I'm always reluctant to upgrade QT when it is working fine.