Is it laggy as in latency? Or as in frames per second? Latency is usually a network issue. (For example if multiple people in my house are using the internet, world of warcraft starts lagging). Could be that the internet you use simply isn't designed for gaming. But I don't think that's the case since you play xbox live unless you have similar issues. (The internet plan as in high speed, or whatever) Some things can help like using an ethernet cable versus wireless.
If it is framerates, everything will be extremely choppy. That is usually when the computer can't keep up with the demand of the game. Or when there are other things running in the background and it is bogging the computer down.
One thing that really kills my online gaming on my PC is when I am using xbox live. My internet now just can't handle it well and it makes one or the other really crappy.