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Charity accepting computers
Oxfam used to take them, but now apparently they don't take single PC's, only job lots. But they may be worth a call.
Just a word of caution, unless you do a clean install of the operating system, it's highly likely that anyone with reasonable computer skills will be able to recover saved passwords / old files / possible personal data etc. etc. You may think you have deleted everything, but there is plenty of stored data all over a PC that you probably don't know about.
Personally I would never give away a PC without doing a clean install, or preferably actually removing the hard drive.
There is a charity shop on Chaussee d'Ixelles (I think it is Oxfam) that do take them - I gave them one about a month ago
I agree with Rupert - we dropped off a couple of old (cleaned down) PCs there recently. Salvation Army may also be interested if you are down in town.