I recommend an excellent article, How to Donate Your Old Computer from Ariane Benefit. Computers are becoming disposable. That old beast from 5 years ago with Windows 98 may be Ancient history, but don't be so hasty when you pull the plug.

The hard drive may only hold 40GB but what's on it? There could be sensitive info, whether it's your financial accounts, legal correspondence, email from cheating spouses, or compromising photos of yourself. And don't forget entertainment and other files, especially if you obtain them from P2P file sharing, iPod copying, IM, or email. Such files could be inappropriate (if you know what I mean) or even illegal. Don't just delete files as they can easily be recovered.

Be sure to wipe the disk with an appropriate industrial-strength program before you donate, sell, give away, trash, or otherwise dispose of your computer. Ariane's blog provides many terrific links for disc cleaners, donations, refurbishers, and recyclers.