The heuristic scanning got a major overhaul, and MSE also integrates with the Windows Firewall. It also has a network inspection module, so it sniffs ahead for trouble when you surf the web (using any browser, but it’s more effective with IE).
Here’s the really cool news: it’s absolutely free for home users, and even free for businesses that have 10 or fewer PCs. If you’re running an older version of MSE, the Windows Update service will upgrade your machine automatically. If not, you can get it from the Windows Download Center.