in some browsers, a form elements value will not be sent with the post if the form element does not have a value attribute.
In your login form:
<form action="" method="post" name="login" id="login">
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr>
<td> </td>
<td><strong>Member Login </strong>(<a href="password.php">forgotten password</a>)</td>
<td align="right">Username:
<input name="username" type="text" id="username" size="10">
Password:
<input name="password" type="password" id="password" size="10"></td>
<td width="50" align="right"><a href="password.php">
<input type="submit" name="loginbutton" id="loginbutton" value="Login" class="button">
</a></td>
</tr>
</table>
</form>
the username and password form elements do not have value attributes, this could be causing the problem
change:
<input name="username" type="text" id="username" size="10">
and:
<input name="password" type="password" id="password" size="10">
to:
<input name="username" type="text" id="username" size="10" value="">
and:
<input name="password" type="password" id="password" size="10" value="">