today i downloaded a movie and the sound is about three seconds ahead. how can i fix it

You can use VirtualDubMOD to split the audio and the video. And then put it back together with a delay on the audio.
It's a bit complicated to use, but there are lots of good guides to be found using Google.

You can download VirtualDubMOD from http://sourceforge.net/projects/virtualdubmod

First of all, this isn't an easy fix for a beginner. It may take some practice.

If your video is out of sync consistently through the entire movie, by the same amount of time, then you are in luck.

If the video gets more and more out of sync, ditch it. It's not worth your time.

1. Easy fix (I haven't done this in a while, but it may be worth a try if you want to watch the video once on your computer and throw it away)...Media Player Classic (free) has the ability to offset the audio so you can sync it with your video. I don't currently have the player, but if you play with the options or look for a guide, you can probably figure it out.

2. Lasting fix (this is what I do):

a. Convert your movie to DVD with a good encoder, i.e. Tsunami MPEG Encoder (TMPGEnc) Plus 2.5 or Cinema Craft Encoder (CCE) basic, both around $50 or less.

b. During the process you will separate the audio and video into elementary streams (not mixed together). The audio should be saved as a .wav file (uncompressed audio).

c. I then use Besweet (free) to convert the .wav audio to .ac3 audio (DVD standard). Besweet allows you to offset the audio. I take a good guess how much the audio is off and give it a try.

d. Then, use a DVD authoring program (I use TMPGEnc DVD author) to put the audio and video back together. If you want, you can give IFO Edit a try...IFO is free and allows you to offset the audio without converting it all in one step, although you won't get the same quality IMO.

e. Preview the DVD file with a media player to see if your audio is now in sync. If not, repeat c-e.

This definitely takes time and effort, although with practice you can get quite good at estimating how much the audio is off. When you offset the audio, you enter an 'audio delay'. So, if the audio is ahead of the video, you need a delay. Enter a positive value. If the audio is behind the video, it is too delayed. Enter a negative value.

There may be a better method of doing this, but I haven't found it yet...even the gurus on video forums recommend methods similar to this.

Good Luck. Probably more advice then you wanted, eh?

P.S. when encoding with TMPGEnc or CCE, it is important to leave your computer alone. These programs take up a lot of juice, and will cause other programs to move very slow. Even worse, using other programs can CAUSE the audio to go out of sync.