SpiderOak is designed to backup select data rather than your entire computer. When selecting your folders/files for backup, make sure to never backup anything you don't recognize. We recommend that you never backup system or application data, such as the System32 folder on Windows computers. This data is time sensitive, and restoring an older 'saved' version could harm your computer.
Also, it's not usually necessary to backup an entire directory (such as 'C:') unless you are certain you want everything in this directory backed up to SpiderOak. If you select an entire directory, you are probably backing up a lot of unnecessary files which will take up space in your account.