The first time that you set the computer up, it prompted you to set a password for the Admin account. It is the same one that you have to enter when installing software or updates. Go to System Preferences > Accounts and turn off Automatic Login, then you will be required to enter the password at start up.
Beware the limitations placed on Time Machine via Filevault as referred to here and with a work-around of sorts here. If you use Time Machine you might want to look at encrypting the backup - or, indeed, any other backup such as that created via "Super Duper! - details of which you'll find by searching the Internet.
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Never ASS-U-ME or you may make an ASS out of ME and an ASS out of U.