I'm facing the same dilemma with my old desktop--its over 6 years old and my new one I got a few months ago.
I thought about donating it, but it's served me so faithfully that I have a hard time letting it go. I feel kind of bad just giving it away to strangers...what if someone abuses it and trashes it? Will they defrag it regularly and open up the case to use an air canister to clean the dust out every couple of months? (Yeah, I can't believe I feel that emotionally about a freaking desktop).
My solution--I'm turning it into a "new" workstation for my husband's business office. Plus it can still run World of Warcraft and a few older games, so its a second low end gaming PC/emulation machine for him.
(Now my old piece of shit Gateway computer that I had 10 years ago, I had no problem donating. I almost cringe at the thought of the person who has it now and has to deal with its daily crashes and bugs).