I've added this post, which I've also put in the Standard Techniques topic, because Windokus are sometimes used with Killers, for example in HATMAN's Old Lace Test and More Lace Practice puzzles in the Other Variants forum.
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 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.