We need to know a lot more if we're going to help you.
What version of After Effects? Have you installed the most recent updates (the 8.0.2 update)?
What operating system?
What kind(s) of source footage?
Has this ever worked before?
What other software are you running? Do you have any third-party effects or codecs installed?
Tell us about your computer hardware.
Are you rendering with OpenGL?
How are you playing/previewing?