Why is my MacBook so slow?

I’ve had it for almost a year, its a black one with a 2 GHz Intel Core Duo processor, 512 MB of memory, and an 80GB hard drive, about 20 gigs of which is free. When more than one or two programs are running, everything is excruciatingly slow and application such as word, firefox, safari, ichat, itunes and even the finder and dock freeze often.