A rectangle start button

I was making a skin about 2 weeks ago under WB 4.6, the start panel was masked with a right radius, making it round like the default windows xp theme. I wanted it to be square and found a way to: I unchecked the "transparency" checkbox in the start button within skin studio, which made my start button rectangle.

But ever since I updated my WB to version 5.00, my start button is now again masked, even with the "tranparency" checkbox unticked.

So how can I remove this hidden mask?