made some adjustments, probably not what you want to do with it but got it working. the problem was that in the beginning of your javascript function you are changing the display properties of some elements by ID, well the problem was that some of those elements were missing so I just threw them in there and it works now. If you use firefox's error console it will tell you this kind of stuff.
Also changed your body font css attribute to font-family since that was also giving me a parsing error.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>TSS SGA Voting System - Login</title>
<style type="text/css">
body {
font-family: Arial, Helvetica, sans-serif;
font-weight: normal;
font-size: medium;
color: #FFFFFF;
margin: 0 0 0 0;
padding: 0 0 0 0;
}
a, a:visited {
color: #FFCC00;
text-decoration: none;
}
a:hover {
color: #FFFF00;
text-decoration: underline;
}
#box #content #loading {
display: none;
}
#box #content #userError, #box #content #passError, #box #content #invalidError, #box #content #usedError {
display: none;
color: #FF0000;
font-size: 14px;
}
#box #content #mError, #box #content #sError, #box #content #tError {
display: none;
color: #FF0000;
font-size: 14px;
}
</style>
<script type='text/javascript'>
function ValidateLogin(form)
{
var error=false;
document.getElementById('userError').style.display = 'none';
document.getElementById('passError').style.display = 'none';
document.getElementById('invalidError').style.display = 'none';
document.getElementById('usedError').style.display = 'none';
document.getElementById('loading').style.display = 'block';
document.getElementById('submit').style.display = 'none';
with(form)
{
if(pass.value==''||pass.value==null)
{
document.getElementById('passError').style.display = 'block';
error=true;
pass.focus();
}
if(user.value==''||user.value==null)
{
document.getElementById('userError').style.display = 'block';
error=true;
user.focus();
}
if(error==true)
{
document.getElementById('loading').style.display = 'none';
document.getElementById('submit').style.display = 'block';
return(false);
}
else
{
return(true);
}
}
}
</script>
</head>
<body>
<div align="center">
<table width="500" border="0" cellpadding="0" cellspacing="0" id="box">
<tr height="30"><td width="30"></td><td id="title">Login</td><td width="30"></td></tr>
<tr><td colspan="3" id="content">
<table align='center' border='0' cellpadding='0' cellspacing='0' id='usedError' style='background:#FFCC00;border:#FFFF00 1px solid; width: 75%; color:#000000; font-size:medium; margin-top: 5px;'><tr><td width='15' bgcolor='#FFFF00'></td><td style='padding-left: 10px;'>Please Enter Your Username</td></tr></table>
<table align='center' border='0' cellpadding='0' cellspacing='0' id='userError' style='background:#FFCC00;border:#FFFF00 1px solid; width: 75%; color:#000000; font-size:medium; margin-top: 5px;'><tr><td width='15' bgcolor='#FFFF00'></td><td style='padding-left: 10px;'>Please Enter Your Username</td></tr></table>
<table align='center' border='0' cellpadding='0' cellspacing='0' id='passError' style='background:#FFCC00;border:#FFFF00 1px solid; width: 75%; color:#000000; font-size:medium; margin-top: 5px;'><tr><td width='15' bgcolor='#FFFF00'></td><td style='padding-left: 10px;'>Please Enter Your Password</td></tr></table>
<table align='center' border='0' cellpadding='0' cellspacing='0' id='invalidError' style='background:#FFCC00;border:#FFFF00 1px solid; width: 75%; color:#000000; font-size:medium; margin-top: 5px;'><tr><td width='15' bgcolor='#FFFF00'></td><td style='padding-left: 10px;'>Please Enter Your Password</td></tr></table>
<form name='login' action='system.php' method='post' onsubmit='return ValidateLogin(this);'>
<table border='0' align='center'>
<tr><td align='right'>Username:</td><td align='left'><input type='text' name='user' /></td></tr>
<tr><td align='right'>Password:</td><td align='left'><input type='password' name='pass' /></td></tr>
<tr><td colspan='2' align='center'><input type='submit' value='Login' id='submit' /></td></tr>
</table>
</form>
<div id='loading'><img src='images/loader.gif' /> Loading...</div>
</td></tr>
<tr height="30"><td width="30"></td><td id="footer"></td><td width="30"></td></tr>
</table>
</div>
</body>
</html>
