While it's highly unlikely that anything will go wrong in the course of an upgrade, we'd still recommend being safe – a new install is a good excuse for a backup! If you're heading for a clean install or an upgrade, you'll want to make sure you don't lose anything valuable in the process.
Safe data is data stored in three distinct places – its original location, and two geographically distinct copies. This means an additional partition isn't really going to cut it, given that it's the exact same physical location as the original data; you need to use a high-capacity external drive, and some kind of online cloud storage. Most free services – Dropbox, Google Drive and the like – only offer a limited amount of space, so only place your most critical files online if you're not willing to pay. Services such as Carbonite or CrashPlan, which generally charge a monthly subscription fee, offer a much more extensive range of backup options, and will generally do all the hard work for you.
Finding the files
The key location to have backed up is your personal folder, which will sit within the Users folder on your main drive. This includes all of your libraries – Documents, Photos and the like – and your Windows desktop. But do be warned it won't cover absolutely everything safely.
There's a small chance you're extremely organised, and you know where every file of a given type resides on your hard drive. There's a much larger chance that everything is scattered in various separate places, so you'll need to dig everything up. Use Windows' search facility to find what you're looking for; open up Computer in an Explorer window and use the search bar at the top to search for, for example, *.jpg to find all of your photos, or *.mp3 for your music. Select the files from your searches and copy them to your external drive and, if you have available space, to your chosen cloud service.
If you have a big enough external drive, there's a way to be ready for any eventuality – completely back up a mirror image of your current hard drive, Windows and all. We'd recommend Macrium Reflect Free (www.macrium.com/reflectfree.aspx) for this; it's a particularly straightforward way to clone a drive, and while it doesn't have advanced features like incremental backups – these are saved for the paid-for versions – it does what you need it to do.