The one by Robertson (mentioned in Silouan’s last comment above) is available online via archive.org at http://www.archive.org/details/stathanasiusonth00athauoft.

Another is that from Schaff’s “Nicene and Post-Nicene Fathers”, with “On The Incarnation” available online starting at http://www.ccel.org/ccel/schaff/npnf204.vii.ii.i.html.

Finally, as for the explanation of the author behind the popular translation (the one with the Lewis preface that Silouan mentions in his entry), the following page indicates that the person behind the moniker, “a religious of SPCK” is a “Sister Penelope Lawson, of the Anglican Community of St. Mary the Virgin in Wantage, England”. More on that, and also an online presentation of this “more modern” translation are at http://www.spurgeon.org/~phil/history/ath-inc.htm

My copy is a 1989 reprint by SVS Press, and indicates that this translation was originally published in 1944 by Centenary Press. It has since passed into the public domain and virtually every edition in print in English reproduces this translation and C.S. Lewis’ introduction. I do know of an earlier translation (1891) by Archibald Robertson, but I don’t have a copy.

Hope that’s helpful :-)