Welcome to the forum.
We'll need more information before we can offer suggestions. This ARTICLE will give you some tips on the information that will be needed.
Good luck, and we will be looking for that info.
This is aimed at Premiere Pro, but may help
A link with many ideas about computer setup http://forums.adobe.com/thread/436215?tstart=0
Work through all of the steps (ideas) listed at http://ppro.wikia.com/wiki/Troubleshooting
If your problem isn't fixed after you follow all of the steps, report back with ALL OF THE DETAILS asked for in the FINALLY section, the questions at the end of the troubleshooting link... most especially the codec used... see Question 1
And be sure to tell us about your computer.
How fast is your processor, how much RAM do you have and how much free, defragmented space is on your hard drive?