Innanzitutto io ti consiglio di rinominare tutte le pagine in .php anzichè usare iframe in pagine html...
cmq volendo risolvere questa specifica situazione ti evidenzio in grassetto le modifiche che ho fatto al tuo codice:
pagina di login in php:
<html>
<head>
<title>Login</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body style="background-color: transparent;" >
<form action="login.php" method="post" target="_top">
<b><font size="1" face="Verdana">Username:</font></b><br />
<input type="text" name="username" size="17" maxlength="40" value="" /><br />
<b><font size="1" face="Verdana">Password:</font></b><br />
<input type="password" name="password" size="17" maxlength="25" /><br />
<input type="checkbox" name="autologin" value="ON" /> <b>
<font size="1" face="Verdana">Login automatico</font></b><p />
<input type="hidden" name="redirect" value="./../forum" />
<input type="hidden" name="sid" value="8d5b5aba99f072d64eec92e924235374" />
<input type="hidden" name="outside" value="1" />
<div align="center" style="width: 156; height: 57"><input type="submit" class="mainoption" name="login" value="Log in" /><p />
<b><font face="Verdana" size="1">
<a href="profile.php?mode=sendpassword&sid=8d5b5aba99f072d64eec92e924235374" style="text-decoration: none">
<font color="#000000">Ho dimenticato la<br> password</font></a></font></b></div>
</form>
</body>
</html>
Codice della tabellina:
<table width="98%" border="0" align="center" cellpadding="0" cellspacing="0">
<tr>
<td width="154" align="center" valign="top"><table width="152" border="0" cellpadding="0" cellspacing="0" background="immagini/bg.jpg">
<tr>
<td width="8" height="15"><img src="immagini/01td.jpg" width="8" height="15"></td>
<td align="center" background="immagini/03td.jpg"><span class="style1">.: Menù :. </span></td>
<td width="8" height="15"><img src="immagini/01btd.jpg" width="8" height="15"></td>
</tr>
<tr>
<td> </td>
<td width="100%" align="left" valign="top"> <iframe src="login_da_hompage.php" width="100%" height="185" align="center"
frameborder="0" marginwidth="1" marginheight="1" scrolling="no" name="I1" allowtransparency="true"></iframe> </td>
<td> </td>
</tr>
<tr>
<td width="8" height="10"><img src="immagini/02td.jpg" width="8" height="10"></td>
<td background="immagini/04td.jpg"></td>
<td width="8" height="10"><img src="immagini/02btd.jpg" width="8" height="10"></td>
</tr>
</table>