[Vulkan]

Можно так:

<script type="text/javascript" src="http://code.jquery.com/jquery-1.5.1.min.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$("#indexform").submit(
	function(){
		if($("#username").val() == "") {
			$("#emptyname").val("Введите логин");
			return false;
		}
		if($("#password").val() == "") {
			$("#emptypass").val("Введите пароль");
			return false;
		}
		if($("#secretcode").val() == "") {
			$("#emptycode").val("Введите символы с картинки");
			return false;
		}
	});
})
</script>

<form action="#" method="post" id="indexform">
		<table border="0">
			<tr>
				<td colspan="2">Введите Ваше имя:</td>
				<td>
					<input type="text" size="15" maxlength="20" name="username" id="username"/>
				</td>
				<td>
					<input type="text" class="text" disabled="disabled" name="emptyname" id="emptyname" />
				</td>
			</tr>
			<tr>
				<td colspan="2">Введите пароль:</td>
				<td>
					<input type="password" size="15" maxlength="16" name="password" id="password" />
				</td>
				<td>
					<input type="text" class="text" disabled="disabled" name="emptypass" id="emptypass"/>
				</td>
			</tr>
			<tr>
				<td>Введите код:&nbsp;</td>
				<td>
					<img src="captcha.php" alt="защитный код" id="imgCaptcha" />&nbsp;
					<input type="image" src="img/refresh.jpg" onclick="return refreshCaptcha();" />&nbsp;
				</td>
				<td>
					<input type="text" size="15" maxlength="10" name="securecode" id="secretcode" />
				</td>
				<td>
					<input type="text" class="text" disabled="disabled" name="emptycode" id="emptycode"/>
				</td>
			</tr>
			<tr>
				<td colspan="2">
					<input type="button" value="Регистрация" class='button' onclick="<?php echo $reglink;?>" />
				</td>
				<td>
					<input type="submit" value="Вход" class='button' id="authsubmit" />
				</td>
			</tr>
		</table>
	</form>