Windows 7 comes with a firewall built in – called Windows firewall. Unfortunately, this Windows firewall allows outbound connections by default. An outbound connection is a connection from your computer to internet. Using this default setting for outbound connection a trojan or some other malicious software can connect to internet and update itself or download more malicious programs without yuo noticing anything. But you can enable outbound protection using these steps:
Now if you launch any web browser you will notice it can not access internet. You will have to manually tell Windows that certain program is safe by creating new rule for this application.