@MissDaphne - I don't necessarily disagree with you that proofreading by hand can be a useful exercise, but I *do* think that it is quite possible to do quality proofreading on a computer screen. It is just a matter of forcing yourself to see the words as you would on a page and to turn off your "computer reading" mode (in which the brain does not read all the words).
I find that the Editing and Reviewing Toolbar (or I guess it's a ribbon in the newer version?) on MS Word is really great for proofreading. You can use "track changes" to show what changes you are suggesting. The original author can then choose to accept or reject each individual change. The only problem you can run into is that if you make too many changes, it gets very messy. It is important for each new edit to be saved with a new file name and for all the old changes to be cleared out before new ones are made.