Our VM plans come with a firewall that you can control through the cloud control panel. Please see here for more information on this: https://help.blacknight.com/entries/22936026-how-do-i-modify-my-firewall-on-my-cloud-vm
You can install an anti-virus program if you want to, however you might want to take into regard the physical requirements of any such program as many anti-virus programs can use up a lot of system resources (e.g. CPU and RAM). If you are installing your own anti-virus program or firewall you will also need to be careful that you do not block yourself out from the server, or restrict email traffic.
For example, when enabled, the default Windows firewall will block Remote Desktop traffic in and website traffic out from a VPS. This will cause you to be unable to log into the VPS or view any websites on it.