The app was installed as an unwanted "extra" along with some app you installed. You have to be very careful when you download and install apps and avoid using sites that frequently include add on apps.
This is an article that offers more background as to what it is and how you got it. Also it talks about an uninstaller offered by AMC which you can find here:
http://advancedmaccleaner.com/uninstallguidelines
BTW: the article says the download link for the unistaller is a link below but it is actually in the upper right corner.
If all else fails you can do some things on your own. Go to Activity Monitor which is in the Utilities folder. Under process name find the entry for Advanced Mac Cleaner it will be called MacDefender, select it and click Quit Process. A dialog should pop up, asking if you are sure you would like to quit the Advanced Mac Cleaner executable. Select the Force Quit option
Now go to your Applications. Find the Advanced Mac Cleaner entry, right-click on it and select Move to Trash. If user password is required, enter it.
Now go to Apple Menu and pick the System Preferences option Select Accounts and click the Login Items button. Mac OS will come up with the list of the items that launch when the box is started up. Locate Advanced Mac Cleaner there and click on the "-" button to remove it.
This will not get rid of all the "garbage" it leaves behind but it should stop the popups.
Lisa