Replacing the registry does *not* make it like a new install. Most Windows errors, slow downs, problems, etc. that require you to reinstall Windows are within the registry itself. Replacing the registry means that you don't get rid of those problems. All you really end up doing is replacing Windows files. If you're going to just do that, you might as well just do a Windows repair from the installation disk because that's all you are doing in the first place. In fact, the Windows repair will actually help MORE because it will also repair some registry data, even though it still isn't as good as a full reinstall.
If you are fixing computers for people, even if it is for free, you really should do a good job at it and not leave them more or less where they were in the first place by relacing the registry. As I said, if you're going to go that route, you're better off just doing the Windows repair function instead. You don't have to reinstall any programs if you do the repair function.