In order to get security updates from Microsoft, you’ll have to have Service Pack 1 installed. If you use the Start Menu "Search Files and Programs” and type version, it’ll give you an option for Show which operating system your computer is using. Clicking that should display “Windows 7“, the copyright date, then if Windows Service Pack 1 is installed, it’ll show that below the Copyright date.
You can find instructions from Microsoft on getting Service Pack 1 here.
You should try to follow the steps discussed so far before doing a Defrag on Drive C:
Since these steps overall may leave a lot of free space on your computer, you should defrag C: drive to condense the contents. I ask you to backup again because a system crash or power loss during a defrag can really mess things up.