Hi,
Use javascript on button click event to open new window, like
ScriptString = "<script language="javascript">";
ScriptString += "window.open('showregistration.aspx?pk=" + strTxt + "')";
ScriptString += "</script>";
ClientScript.RegisterStartupScript(this.getType(), "<msg1>", ScriptString);</msg1>
Hope this helps...
Enjoy coding...