hi Sometime back i also faced same issue, i solved by following method
ScriptManager.RegisterStartupScript(Page, Page.GetType(), "Success", "fnPayment()", True)
fnPayment() javascript
function fnPayment() {
var windowFeatures;
windowFeatures = "type=fullWindow,fullscreen,toorbar=no,status=yes,scrollbars=yes,left=0, top=0;";
window.open('aspxtest.aspx?reg=' + intReg , '_inst', windowFeatures);
}
hope this may help you