1. Files/Programs ?2. photos/music ?3. windows media player playlists ?4. Norton Internet Security ?5. browser favourites ?
If everything goes right, nothing. If you do an upgrade and not a clean install, then all your files should be waiting for you when the upgrade is complete and Win 10 launches for the first time. However, there is always that poster on this forum who comes in to lament that something got lost in the upgrade. Stuff can happen…if your PC\’s hard drive is weak, trying to install a new OS could put it over the edge and it could fail. So, my recommendation is to back up. Whether you upgrade to Win 10, or decide to keep Win 7 or 8.1, you really should have all of those things backed up anyway. Get it on a cloud service like OneDrive or Dropbox. There are specialized backup providers like Crash Plan and Carbonite, that are reasonable priced. You may have an old PC with plenty of hard drive space sitting in a closet somewhere. There\’s even the old school blank DVDs and USB drives. You should be doing this anyway, but if something should go wrong with the upgrade, you are protected. And at least you have one good backup of your system if your Hard Drive should crash down the road, or if your PC would get damaged, lost, stolen, or whatever.