Hi, I had this same problem when I installed today, I installed on a different directory to windows and I am using vista. The fix for me was to edit file vhost.php \UniServer\home\admin\www\vhost.php on line 22 // Path to hosts file
$hosts_sti = $_ENV["windir"]."/system32/drivers/etc/hosts";
I changed it to // Path to hosts file
$hosts_sti = $_ENV["windir"]."C:/Windows/system32/drivers/etc/hosts";
Also vista the file is called 'hosts' not 'hosts.txt' Maybe this will help you? Good Day.