function trim(str)
{
    return str.replace(/^\s\s*/, '').replace(/\s\s*$/, '');
}

function validate(form)
{
    var username = trim(form.username.value);
    var password = trim(form.password.value);
    var valid = true;
    
    if (username == '') {
        form.username.style.background = '#ffffcc';
        valid = false;
    } else {
        form.username.style.background = '#fff';
    }
    
    if (password == '') {
        form.password.style.background = '#ffffcc';
        valid = false;
    } else {
        form.password.style.background = '#fff';
    }
    
    if (valid == false) {
        alert('Proszę wprowadzić dane');
    } else {
        form.submit.className = 'submit disabled';
        form.submit.disabled  = 'disabled';
    }
    
    return valid;
}