You could make a new user account set it as admin then once on the new account delete the old one with all the files or copy some files you want over before deleting it.

You could run a standard system restore, but I recommend creating a system restore Point before you do anything just in case anything bad happens(usually nothing happens).
If you have a USB flash drive around you could make it a bootable windows drive and wipe everything that way.
If you install windows over I'd make sure that you have the license key anywhere and then start the process.

When you have done this and you know no bad things happened, you can use Disk Cleanup to delete the old Windows files if they are still there.

If you want, you can format all drives that you want and make it as a brand new drive. This is done with a Windows bootable device like a USB Flash Drive if you may. You will need a license key to be able to reinstall windows and you need a functioning Windows bootable device with the same version of Windows that the license key supports, this is usually not needed, but you could run into issues on install and activation.