Here's an option which offers up to 7gb of free storage, tho you can buy more for very cheap. It will also allow you easily to Sync your files to all of your devices. It's all explained here: Sync, Backup and Store your Files to the Cloud with Skydrive - Windows 7 Help Forums.
You can upload the files manually from your SKydrive account you create, or install the Skydrive app to drag anything you want backed up (and later sync'd) to its folder. Just be sure to Unlink it in Settings before you reinstall so files are not deleted when the folder is.
You can also create a data partition on the end of your HD to store your data. As long as you don't touch it during reinstall, just deleting and recreating the C partition instead, your files should be safe there. If you do this I'd use both methods. It's always best to have another HD or external.
Here are also steps to get a perfect Clean Reinstall - Factory OEM Windows 7
which work equally well for retail.