Can't AllowsTransparency in windows XP

Question

Hello, 

I can't set AllowsTransparency="true" in windows XP (Windows 7 is OK)
http://i1120.photobucket.com/albums/...kromeo/XML.jpg

Error:
http://i1120.photobucket.com/albums/...o/thongbao.jpg

I'm sorry, my English is very bad.

Please, help me. Thanks.