Microsoft Security Essentials doesn't get in your way. It runs quietly in the background and schedules a scan when your computer is most likely idle. You only see alerts when you need to take action.
You can't use Microsoft Security Essentials with Windows 8, but you don't need to - Windows Defender is already included and ready to go. But if you're looking to protect a PC with an older version of Windows, you can use Microsoft Security Essentials to help guard against viruses and other malicious software.