Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(startRequest);
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endRequest);

function startRequest(sender, e) {
	//disable search button during the AJAX call
	document.getElementById('ctl00_cphSubContent_btnRegister').disabled = true;
}

function endRequest(sender, e) {
	//re-enable the search button once the AJAX call has completed
	document.getElementById('ctl00_cphSubContent_btnRegister').disabled = false;
}