My old computer died last week. My writing companion for many years, its demise and replacement left me with a toxic metal, plastic and silica corpse. One way you can dispose of an older working computer, is to sell it. I live in a college town with plenty of impoverished students glad to get a computer for a few bucks. Another place to sell your computer is to second hand stores and computer dealers.
Many corporations give their old PC's to schools.
Strangely, they have strict rules about what machines they accept. So contact your local school board to find out how to do this. Sometimes you can get a receipt suitable for tax purposes. If your school does not want it, try local charitable organizations. This helps them use less of their cash donations to support their administration.