What may be an old computer to you may be a treasure to someone else. Linda Musthaler lists some good donation options here, along with business steps for tracking and accounting for those donations.
Don't listen to the guy saying “schools need computers, so let's take it to the high school.” Schools may need computers, but more importantly they need consistency in their units for management and cost savings.
Local computer companies sometimes get involved in helping recycle systems by the handful rather than the truckload. Take a look at Computer Experts in Indianapolis, and how they helped run a Drive-Through eCycling event. They also include a link to a program to safely erase your hard drive called DBAN. This step must be done before you donate your system.
Well, I guess if you want your personal files and bank account information to be donated along with your system, you can skip that step. Most people prefer to keep their private data private and erase the disk. Windows doesn't include any tools strong enough to completely erase the disk beyond easy recovery, which is why you need a program like DBAN.