I have been the recipient of several donated computers for a school. There is a Microsoft programme called Fresh Start for schools to get copies of software for donated pcs. Then there is a refurbishment programme (MAR) where you give the pcs to an agency who refurbish them and sell them on to charities. (Details on ).
IN my experience the best thing you can do is donate the pcs and original software to a deserving cause. The biggest problem for recipients is that donors (quite rightly) wipe the hard disks and forget to include the operating system and drivers software with the donation.
It is completely legal and recommended by Microsoft etc to hand on this software. The only formality is a statement that the pcs are a donation to the recipient.
Try your local school. However IT budgets have increased recently. Many community centres, adult literacy, etc would be glad of them. My own organisation would also take them happily. PM me if you are interested.