That's not much to go on. We need details because AE is working pretty darn well on compatible systems with standard footage. Make sure AE is up to date (latest build) and make sure that you are not running a bunch of other apps in the background while trying to use AE, and finally, make sure that you are using a standard format for the footage.
If that does not fix your problem then we need details.
Did you ever get your previews working? Let us know if you still need help.