And if Ben's suggestions don't help, might I recommend you start telling us stuff about your machine? Have you updated your AE to the latest? (I think CS4 was version 9.0.3 or something.
What version of Windows are you using? Have you updated it recently?
What are your computer's specs?
Have you installed any video codecs recently?
When did you first notice this issue?
Has it worked before?
- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')
No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.