You may have deploy an Azure VM but forget to install antivirus security extension during provisioning. If yes, then you may refer to below step on how we deploy Microsoft Antimalware f9or FREE.
Windows guest operating system from Win 2008 R2 SP1 or higher
Install Window Azure VM Agent into VM. Download from here.
[ Configuration ]
Use Windows Azure Powershell to execute below command
$servicename = "MS4U-VM1"
$vm3 = Get-AzureVM –serviceName $servicename –Name $name
# Get the VM
# Add Microsoft Antimalware Agent to the Virtual Machine
# Update the VM which will install the Antimalware Agent