It is called Microsoft Security Essentials and it has been around since 2009, succeeding Windows Live One Care.
It is free and easy to use.
It is faster than other paid antiviruses.
It is good for low-end computers with little memory or slow processors such as netbooks and laptops under $400.
It does not have an anti-spam protection.
It does not scan within outlook pst files. This means it may not be effective to remove viruses that are already on the system.
Microsoft Security Essentials does not actually scan incoming e-mails. But it does scan e-mail attachments and files when you open them. This makes it much faster than other antiviruses. Unofficially, Microsoft says that scanning e-mails on arrival is not necessary.