Answer:
The main difficulties in arriving at an overall total arise from major uncertainties as to the number of Soviet soldiers killed in World War 2. The figures for military dead for the Soviet Union range from 8.8 million to 10.7 million.
See the link to the Wikipedia article, which (as accessed on 13 April 2010) gives a worldwide total of military dead ranging from about 22.6 million to about 25.5 million (and an overall worldwide range, including civilians, of 62.4 million to 78.4 million).
Please see the link and related questions.