Installing:

Run the installer and click next, agree to the license agreement, click validate and then install. After the install has finished leave the "Scan my computer for potential threats after getting the latest updates" box checked and click finish. A Microsoft Security Essentials window should come up and start downloading updates. After it gets the latest updates it will do a quick scan.

Updating:

The program should keep itself upto date but it is a good idea to manually update befor a scan. Open Microsoft Security Essentials and click the update tab and then the update button.

Scanning:

To run a scan open Microsoft Security Essentials and under scan options click full scan, then click scan now.