Farewell, old friend! Your computer has been retired and needs to be e-cycled.
If your office's trash disposal contract has a recycling option, that would be the first method we'd suggest.
If not, we'd recommend bringing it to an Office Depot, Staples or Best Buy, all of which should have recycling programs. If the store is out of your way, maybe call the store first just to be sure it's offered there. Office Depot/Max may charge a little so consider that your last option.