I'm sorry but you haven't given us nearly enough information to help you.
As the announcement at the top of this forum says, we'll need to know, among other things:
What operating system are you using? Have you gone to Windows Update and ensured you have even the non-critical updates? Have you ensured you have the latest version of Quicktime from Apple.com?
How fast is your processor and how much RAM do you have? Have much free, freshly defragmented space is on your C drive?
What are your project settings? What type of camcorder is your video coming from and how are you getting it onto your computer?
Have you created an Adobe ID and registered your product?
While you are gathering up the info, you might pick up some tips on getting your computer ready for an editing session, from this ARTICLE. Also, if you have Win7, be sure to see the link to Black Viper's Win7 tune-up page, down-thread.