Dropbox by default does not backup your computer. Now you could do this a few ways. I think the easiest would be to use something such as Acronis, or Windows Backup and save the backups to the Dropbox folder (when you install the Dropbox software on your computer there is a Dropbox folder that is created and synced to the cloud)
I think the real issue with that is going to be the space limitation. Might want to look at something such as Back Blaze
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