< html >
< head >
< script language="JavaScript" src="
https://ajax.googleapis.com/ajax/libs/jquery/1.5.0..."> script >
< script >
function check() {
if ($('#input').val() != '')
$('#button').removeAttr('disabled');
else
$('#button').attr('disabled','disable');
}
< / script >
< / head >
< body>
< form >
< input type="text" onkeyup="check();" id="input" />
< input type="submit" id="button" disabled="disabled"/>
form >
Пробелы убрать.