Almost everyone that plays Halo PC does not understand the concept of leading your shots. In a game like Halo PC, with its notoriously laggy netcode, you have to lead shots. The idea is fairly simple and easy to master.
What you have to do is aim a little bit ahead of where your opponent is. For example: if your opponent is moving to the right at a constant speed, you have to aim just a little to the right of him to actually hit him. It just takes some practice. If you get good at it, you can get just as good as you would be without leading. However, in games with a huge amount of lag (anything over 150 [ping] and it becomes pretty darn tough), it's pretty useless. Try to stay in games with a ping below 150. I feel most comfortable with games that have a ping of about 80-100.
One good method for guaging how much you have to lead your shots is to throw a grenade as soon as you spawn for the first time in the game. Count how long it takes for it to actually fly through the air after you see your arm throw it. That is my best method. Happy Hunting!