So you have just bought a new personal computer for your home (rather than for a workplace or as a server) and want to secure it (including protecting it from viruses and spyware). Privacy (including encryption, cryptography and anonymity) is a part of security but broad enough to need covering separately. Think of Privacy as the flipside of the coin. Making backups of data, defragging, system restore points are only indirectly related. Backups can actually make your data easier to steal and retrieve.
This article assumes you wish to use a network (such as the internet), share files on thumb drives and that your PC might be physically accessible to others. If none of those apply, then your many of these steps may be redundant as your PC will already be quite secure.