var handleUsernameKeypress = function(e) {
	if (this.value.indexOf(' ') != -1) {
		this.value = this.value.replace(' ', '_');
	}
}

var usernameOnload = function() {
	var elUsername = document.getElementById('txtUsername');

	if (elUsername) {
		elUsername.onkeyup = handleUsernameKeypress;
	}
}

if (window.onload) {
	var oldonload = window.onload;

	window.onload = function() {
		oldonload();
		usernameOnload();
	}
} else {
	window.onload = usernameOnload();
}