Well, I'm no noob, and I've been playing Halo PC online for quite some time now, and I guarantee you that there are mods out there. I don't know where you get them, and don't want to know. Frankly, I hate when people with mods join in a Halo CTF match because it throws off the balance.
When to suspect a cheat mod:
1. When you are an unreasonable long distance from someone, driving in a hog or flying a banshee, and someone picks you off with a pistol over and over again (and if you pay attention, you'll see that they are doing it to others, and always with just a pistol).
2. In a full 16-person match, you see an average of 20 kills and 15 deaths per person, except for one person who has 65 kills and 0 deaths.
3. When you blast someone 6 or 7 times with a missile, or close-up shotgun straight to the face, and the live through it (and while you're laying there dead, you notice others shooting them, and they live through that as well).
4. When someone openly admits that they are using a cheat mod (and some people have no shame and don't have a problem admitting it).
From a programming perspective, it isn't really difficult to fool the Halo server into thinking the client is legitimate. A mod basically does some of the work for the player (such as aiming) transparent to the server.
The people who use these mods are one of the lowest forms of Halo players, 2nd only to psychos who like to TK for fun.
