If you are looking for simple virus protection, MSE is a 4.5MB download with a clean interface and small memory footprint. If you're already fairly smart about using your computer safely, Essentials will likely give you just the amount of protection you need. MSE stays up to date automatically, and can be scheduled to run at any time or interval. It has a handful of other configuration options, but is otherwise straight to the point.
Much of the functionality found in mainstream AV products is redundant, considering that Windows already has a firewall built-in and you're probably behind either your business's or broadband router's firewall. Additionally, most browsers have phishing detection features to help protect Web surfers from being duped.
MSE is also an excellent compliment to Windows 7. Regardless of Microsoft's tattered image regarding security, Windows 7 is a pretty secure OS, and a suite of security apps can do as much to hinder performance as to protect the PC.