Make sure you play on Punkbuster enabled servers and you should be fine. Call of Duty 4 is great on the PC mostly because of the modding community. You'll find new game modes and tons of new maps. You'll even be able to take part in 64 player servers, but those tend to be full of chaos.
Populated at weird times and prone to infuriating lag comp issues. I'd only recommend it on PC over consoles if you could play in the early afternoon EST. Like right now there's less than 5k people playing but it peaks at around 20k around lunchtime.