Burn then to CD/DVD or copy them to a USB stick or external drive.
Most of your files should be under c : \ users \ your login name \
If you have game saves, it is a good idea to google around for the locations. If you use steam, take a copy of the steamapps folder then you can just past it back over.
With any luck you will not even need these backups(normal upgrades should migrate them over), but it is better to have it and not need it than the other way.