Throw it away (except for the case maybe)!
You can buy an equivalently powerful machine for a low price and save a lot of energy. The cost of the energy will eventually outweigh that of the new pc and then there's the whole altruistic aspect of saving energy... Donating it to charity is a bad idea for the same reason. Buying a new one is also better for our (or China's ;) ) struggling economy.
Instead of just claiming stuff, would any of you be kind enough to do the math? No? All right, I'll do it for you:
An old PC probably uses about 200W. If it runs for 10 hours on 250 days a year, it uses 500kwh, which - where I live - cost about 100€. Today, there are better systems that draw maybe 25W, so they save you about 87,50€ per year.(of course, if you use it as a server that is always running, you just about doubled that figure.) Now I'm pretty sure you don't need that much energy to produce one of these, but somebody might be kind enough to point out some actual data instead of just downvoting this.
And how would the economic growth be false? Old stuff being replaced by better stuff doesn't imply there's no real growth.