Solucionado :D
Código PHP:
$wea = mysql_query("SELECT UserAcc FROM ActivesAcc where UserAcc ='$Us'");
$row = mysql_fetch_row($wea);
$wea2 = mysql_query("SELECT PasswordAcc FROM ActivesAcc where PasswordAcc ='$Ps'");
$row2 = mysql_fetch_row($wea2);
while ($Us == $row[0])
{
$Us = Gen_Cod_Aleatorio();
$wea = mysql_query("SELECT UserAcc FROM ActivesAcc where UserAcc ='$Us'");
$row = mysql_fetch_row($wea);
}
while ($Ps == $row2[0])
{
$Ps = Gen_Cod_Aleatorio();
$wea2 = mysql_query("SELECT PasswordAcc FROM ActivesAcc where PasswordAcc ='$Ps'");
$row2 = mysql_fetch_row($wea2);
}