윈도우 투명

링크1 : http://www.devpia.com/Maeul/Contents/Detail.aspx?BoardID=50&MAEULNo=20&no=842514&ref=842491

링크2 : http://msdn.microsoft.com/en-us/library/ms997507.aspx

링크3 : http://www.codeproject.com/KB/GDI/pxalphablend.aspx