Donating without an OS (or the password) would lessen the value of the donated box. (Even though there is way to reset the forgotten password to empty.) If you have the original installation disk, then reformatting and reinstalling would be a way. But if you have the OS sticker and the registration key, the lack of the original installation disk is not such a big problem. You can get a grip on an installation disk some way.
I once bought for wifey a second hand computer which had been reformatted and the OS new installed. After a time the disk developed a failure, the OS would not boot any longer and the contents had to be rescued with the help of PhotoRec (many thanks, Christophe GRENIER!). It was a though work costing several days, but the data (and my a... that is the rest of my authority) were saved.
The files were rescued without the name and the directory structure just the contents and type. I had to wade through them to create some order before handing them over to wifey and it turned out, that many files of the previous owner were also rescued! Even though the disk had been reformatted and the OS new installed!
A sad story took shape. The wife of the previous owner got more and more sick, she even had to come under the trusteeship (guardianship) of her man. Then she died, then he also got more and more sick, then he also died and the daughter sold the inherited stuff including the PC, which came to a small electronics repair and second hand shop, was sold to me and our story began.
Take care!
P.S. I once had a rescue CD. It came with a computer magazine. It might have been in the good old XP days. It was essentially a Linux system with a special boot menu (but you did not need to know or note this) with various useful functionalities instead of the usual Linux installation. One of them was the resetting of the Administrator password. I once had to apply this to the PC of my father (who had forgotten the password) and it worked like a charm.
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Iván József Balázs
(Hungary)