I'm sorry. You've not given us nearly enough information so that we can help you.
What is your operating system? Have you download the version 3.02 update?
How much free, defragmented space is left on your hard drive?
Have you got the latest Windows updates (even the non-critical ones) as well as the latest version of Quicktime?
And, most importantly, what type of camcorder did your video come from and how did you get it into your computer?
Welcome to the forum.
Look closely at Steve's suggestions for the necessary info - especially the amount of defragmented free space available on your HDD. Most similar issues relate to PrE not having enough available HDD space to work with. Besides the output files, it needs a great deal more space for the working files - more space than most folk realize.