Making a backup of the system image in windows 7
A Windows backup will create a set of files and folders that will not erase your backup media. Why? So that you can store multiple backups on the external disk.
Another method of backing up is to use a WinPE disk, DISM and capture WIM files of your Windows partitions. You can then use DISM to mount your WIM files on your restored computer to restore your personal files.
There are many ways to backup/restore...