I should have said Although you could backup to disks with some programs it would be unwise due to the amount of disk required and the possibility of the disks not working. You have an external hard drive that you can create backup copies onto, just create/name folders for the backups so you know which backup belong to which computer.
If you have a Seagate or Western Digital hard drive - either in the systems or as the backup drive, you can get their free (restricted) version of Acronis. It will do complete drive clones and images, just as the full commercial version.
As BRYNIT says, If you back up a large amount of DATA to a set of DVD's, and just one of them fails, you lose everything.
An external Harddrive (USB)is your best option, you could split it into 2 drives and use this for the 2 machines. However as you are mainly backing up documents and settings, not whole systems, I would recommend you copy the actual documents to the back up drive.
this is a much more reliable system then DVD's.
An easy option is to do the following. From the Desktop, right click on My Documents and select properties, change the default to the new drive, and select move.
This will move all your documents to the new drive / folder. In the event of your main harddrive failing, your documents will be safe on another drive. Terry