If you are getting that error *after* you've rendered the sequence then that means most likely that the harddrive can't keep up. Are you rendering to a drive that is dedicated to just media (good) or are you rendering to your system drive that holds your OS and applications (bad)? What format of video are you working w/specifically (you can select a clip in the timeline, hit command+9 and you'll get all the relevant info) and what type of sequence/timeline are you working in (w/the timeline active hit command+0 to get all the relevant info).
Lethal