Windows Defender (formerly known as Microsoft Security Essentials, WD replaced MSE in Windows 8) is still the
worst possible AV-antimalware you can use on Windows systems in comparison tests.
Microsoft's WD solution here falls under the "
better than nothing" category, but one of the more common user comments I've seen for that product is simply that because it's made by MS on Windows for Windows, it tends to give a lot fewer compatibility problems than other AV programs. (And it's free, natch.) For power-users who already have other aspects of their usage locked down against attacks (including worms), it's probably fine, but for average users who can be tricked by various things on the internats, a real AV/antimalware solution is better.