Obviously, a computer that never accesses the Internet, has only one user and is only used for writing letters to friends and family is more secure than a computer that is shared by multiple members of the household, possibly hosting a personal web site, used for downloading files or participating in online chat sessions. Regardless of the intended use, the three basic keys are to install anti-virus software (and keep it up to date), never open files from sources you dont know and keep your system properly patched against known vulnerabilities.
Beyond that, there are a few other precautions one can take to try to ensure the security of their computer.
1. Use a firewall. This can mean a few different things and each offers a slightly different level of protection. You can have a hardware firewall- such as those contained in home DSL / Cable Modem routers, a 3rd-party software firewall applications, or on some versions of Windows you can implement a firewall within the operating system.
2. Use tough passwords. Using your last name or the name of your dog as your password and never changing it poses a security risk. First of all, many pieces of information about you can be learned by diligent hackers. Items such as your name, the names of your children and other personal information should not be used. To be strong, it is best if your password contains letters, numbers and special characters (such as * or %) and does not contain an actual word. You should also change your password frequently- at least every 30 days.
3. Rename Administrator. Because the Administrator account is created by default it gives a hacker 50% of the information they need to access your computer. All they have to do then is crack the password. To make things more difficult, it is good practice to rename the Administrator account. You can call it anything you like.
4. Create a Guest password. The Guest account is also installed by default and it is installed with a blank password. Generally, the account is disabled and does not pose a threat. However, some hackers and hacker utilities can enable the Guest account. With a blank password they can then get in to do more damage. It is wise to assign a password to the Guest account, but leave the Guest account disabled.