private void Form1_Load(object sender, EventArgs e)
{
this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None;
this.BackColor = Color.Red;
this.TransparencyKey = this.BackColor;
}

http://blog.csdn.net/haukwong/article/details/6853874

C# 在windows form实现不规则窗体&透明窗体