A useful feature of Windokus is that, as well as the four given windows W, X, Y and Z in the diagram below, there are five hidden windows A, B, C, D and E. Each of these windows must contain the numbers 1 to 9; the normal rules for rows, columns and nonets also apply.
It's easy to work out the hidden windows A and B, which are the remaining groups of cells in C234 and C678, similarly hidden windows C and D which are the remaining groups of cells in R234 and R678. Then there are 9 remaining cells R159C159 which form the final hidden window E.
Thanks Simon for explaining these hidden windows here.