Do you have children? Do you have other users that rely on your computer?
If yes to either, then you should create multiple accounts to decrease the chance of viruses/spyware from malicious software. By default, a machine with one user automatically loads that account as the "Administrator". An Administrator account allows all software to be installed despite it being "good" or "bad" and the computer's registry to be changed without any notification; the combination of these two can be very detrimental for a computer since it allows malicious software to have free-reign over your computer.
For your personal account, you can leave it as the Administrator and it's recommended that you password protect it. For all additional users, you should make them Limited accounts and they can password protect themselves later if they choose. A Limited account cannot modify the registry or install most software programs without Administrator rights. Even though this is not a 100% way to block viruses/spyware, it will drastically decrease the chances.