Keep your old computer out of landfill - Lifehacker Australia
First up, you may be able to extend the life of your computer by upgrading some of its components, or you could repurpose it (or bits of it) as a home server. Or you could give it to a friend or relative (make sure it’s powerful enough/stable enough to meet their computing needs or you’re not really being that helpful).
If you can’t repurpose the computer, recycling’s the next option. Extreme Tech’s just posted a helpful article on ways to recycle your hardware, which suggests googling “computer recycling” and your city to see what options come up. Freecycle is another option - a well established email list in which members can give away their unwanted stuff, or see what others are giving away.