Backup selected files only

FreeFileSync is a folder/file syncing program that will do what you want. It will compare your Documents folder on your computer to a folder you select on your backup drive. Any new or changed documents on your computer will be copied to the folder on the external drive. If a document has been deleted in the computer's folder, it will be also deleted from the folder on the external drive.

I would also suggest you put your Documents folder in a separate partition on your computer's HDD instead of leaving it in the C: partition. That way, you can easily make an image of your C: partition which will allow you to easily restore it should something ever go wrong with it (the OS is far more likely to need restoring than your documents). I highly recommend using Macrium Reflect Free (do not download from the homepage; this page is safer to download from) for making images instead of Win 7's backup tool. Here is a good tutorial on how to use Macrium Reflect. Moving the documents folder to another partition is a bit involved and challenging if you have never done it before so, if you want to do so, ask us in a new thread and one of us can walk you through it. You will need a backup copy of the documents folder on another drive anyway so I suggest getting that worked out first.