I have an ethical problem with this: the most common things for people to do with their old, unwanted computers are:
1. Throw them out
2. Sell them on eBay
3. Donate them to a charity
(1) has already been touched on.
As far as (2) and (3) are concerned, you can bet the people receiving the computer are going to be even less computer-savvy than the person getting rid of it. How are they going to cope?
I think the person getting rid of the computer has the same obligation to clean it up that, say, a nuclear power plant might have to decontaminate its waste products.
Car out of oil? No problem! Buy a new one.