Video editing

I have a 20 minute .avi video that needs to be edited down to 10 minutes for use on youtube. I can do the editing, but what I need specifically is to "fix" the video because for some reason the video plays only 24 seconds but if you hit the rewind slider before the 24 second mark, it will keep showing snippets of the video but not more than 24 seconds at a time. If you keep rewinding the video before the 24 second mark you can almost play the whole thing. For example, it you hit play, it will start the video, but only play 24 seconds. If you rewind back to the 3 second mark, you will see the video somewhere after the initial 24 seconds. As long as you keep on rewinding back to the 3 second mark while not allowing it to hit the 24 second mark, it will play the video in it's entirety. I have tried this on 2 different computers and have gotten the same results. The file is 789,152 kb and I have it stored on an online folder that can be accessed by ftp. The file is too big to upload.