DUDE, what's wrong with my 2001 Ford Focus ZX3?

Recently I've had a noticeable problem with my car....Every time I start the engine (only when its cold) it vibrates erratically. It does improve as it heats up, but I can still feel it shaking. I have replaced the spark plugs, and air filter, and I poured a bottle of fuel injector cleaner...the problem persists though. There is no check engine light illuminated, what could it possibly be???