I can't imagine that there are any charities that would want that. I'm surprised you even found charities that want whole PCs. Do they specify what kind of PCs?

I was under the impression that schools and charities didn't want old computers anymore. It's hard for them to refurbish a bunch of one-offs, especially when they can get professionally-refurbished systems bulk-ordered for very little. I could order a dozen Core 2 Duo Dell Optiplex computers for under $1000, and they'd come with fresh Windows 7 licenses. I think a charity would consider that a better use of their time and money, rather than taking one of my client's ancient XP towers they've had since 2002.