Windows firewall (Windows 7)
A firewall is not the same as an antivirus programme. A firewall can help prevent hackers or malicious software (malware) from gaining access to your computer through a network or the internet. A firewall can also help stop your computer from sending malicious software to other computers.
If you are using a machine on the Managed Windows Service then Windows Firewall is enabled by default. You can check it is enabled by clicking on Start > Control Panel > Windows Firewall