There are a number of programs available to convert your video. The best place to search is Cnet.com, which has shareware and freeware for just this purpose. But be careful--some free programs carry spyware or trojans, which can infect your computer. When you download one, make sure you scan it with your virus protection and/or spyware program.
Once you've picked a suitable program and installed it, read the instructions. it's always a good idea to get a feel for the program instead of jumping right into it. Make sure you understand the directions before you alter your movie; you put a lot of hard work into it, and you should give it the best chance possible for success.
Last but not least, make a copy of your original. I can't stress this enough; if the program does something odd to your movie, you may not be able to fix it. Use the copy to test the program, and if it works, you'll simply have two versions. This will also come in handy if you need the original for anything in the future.