While this has good sentiment, I feel like a lot of nonprofits and charities would rather you volunteer your physical support rather than get rid of malware, which nowadays a lot of people are capable of doing, and they might be apprehensive to let you go and change stuff in their computers if they have it laid out in some idiosyncratic way. But I think the best option would be to ask the nonprofit themselves, what the best way you could be helping is.