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#21
It's always a good idea to back up your files before an upgrade, you could also make a system image on an external HDD to restore the PC as it was before you started. In fact, it's a good idea to have a recent system image at all times. Macrium Reflect (free) is the method most here recommend, but Windows own 'Backup & Restore (Windows 7)' can also make a system image.
Having said that, backups are just a sensible precaution. An in-place repair upgrade should keep all your data, documents and installed apps.