As in a backup computer to use, or a backup of your data?
If the data, you can backup your hard drive to an external after you install your preferred basic programs.
If you use multiple computers for work/file management, you can get an Office365 subscription that comes with Microsoft OneDrive storage to keep designated folders from your computer on the cloud. Windows can get you 5GB free, 50GB for $2/month. I don’t know the storage limits when you lump it in with 365 subscriptions.
Past that, you can create a file server on your home network to connect multiple computers to for storage and media streaming. You can either dedicate an old desktop computer to this task, or you can purchase a dedicated hardware solution to do this for you.