After Effects is a very pr

After Effects is a very processor and RAM intensive program. If you do not have at least a dual core processor and 3GB of RAM it will be a real problem. There is also what is called a RAM preview, is that what you are using?

Here are some help articles on previews in After Effects

http://livedocs.adobe.com/en_US/AfterEffects/8.0/help.html?content=WS3878526689cb91655866c1103906c6dea-7ec9.html

http://www.5min.com/Video/Adobe-After-Effects-7—How-to-Preview-Audio-and-Set-Up-a-RAM-Preview-85439546