How do I reduce the preview rendering quality?

Hi there,

When I try to preview my video (in the little preview window in the top right), the playback is always laggy.

I know the computers are the best, but I have to make do with what I have.

Is there any way to reduce the playback quality so that it will play without the lag. Although upon finilising my video, I dont want to remove any quality.         

Thanks

Nathan