What to do with Old Computer?

During your computer's life, you can make it greener by using it wisely and by extending its life; beyond that, your main impact is what you do with the computer at the end of its life. Using your computer wisely means treating it like any other appliance and turning it off or putting it to sleep when it's not in use. (This can include just parts of your computer--for instance, if running it as a music server, you can shut the monitor off.) Extending the life of your computer means upgrading it or adding peripherals to keep it useful years longer than it normally would be. This is easier for desktops than for laptops, simply because desktops are much more modular. Many brand-name machines are specifically designed to be easily upgradeable, and all systems cobbled together from third-party components (known as white boxes) are by their nature reconfigurable.

Even older systems where the CPU cannot be upgraded further and there's no room for more RAM can sometimes see great benefit from just replacing the video driver, given the more multimedia-rich environment we are now in. If the computer is absolutely not useful as a working computer anymore, it still may be good for a niche application (like the music / entertainment server mentioned above) where it could eliminate the need for a separate CD player, DVD player, etc.


Reselling and Donating
The best way to get rid of your computer is to sell it (or give it) to someone who still finds it useful. Doubtless there are a few older machines--especially ones with large CRT's--which are such energy hogs that two or three more years of electricity use would cause more impact than manufacturing a new replacement; but the only way to know that is to do a life cycle analysis. Most of the time it will make sense to keep old machines running. The two most popular sites to sell used electronics are eBay and Craigslist, simply because they are so popular for all person-to-person selling. SecondRotation is a company that will buy your old cell phone, GPS, digital camera, or laptop (Apple only, so far) for a fixed fee, then resell it. A quick check on the value of my MacBook Pro showed that I would get more selling it on eBay, but that may not be true for some of the other product categories, and they offer the security of a known buyer/seller.

If your computer is too old to talk anyone into buying it, you can donate it. Free Geek has, as they say, been "helping the needy get nerdy since the beginning of the 3rd Millennium". They and other similar groups like Youth for Technology Foundation and Network for Good take old computers, refurbish them if necessary, and give them to low-income schools (both in industrialized countries and developing countries.) TechSoup has a good list of tips for donating a computer.
Src: http://www.worldchanging.com/archives/007587.html