User Accounts. They type of account you use on computer can make the difference between being infected and/or hacked and not. Do you know if your account has full administrator access or not? Here is an article on why it is best to Use a Standard vs. Administrator Acct.
But something even more important in some aspects of the type of user is account is whether you are using auto-login or not. The reason for the importance is that if you have this feature enabled and someone steals your system they have full access to all your information. Windows Vista and Windows 7 do not have an easy to enable this so if you have either of these operating systems and it is logging in automatically I would disable this feature. If Windows XP is automatically logging in check to see that you do have a password set, otherwise click on the your operating system to find out how to disable. MAC Windows XP
One change you will want to make on Windows, all versions, is to set a password on the administrator acct. This is done through in the Control Panel in the same place you would add or make changes to your user accounts. When you set the password you will want to make sure it is something you will remember, and if you have a safe or cabinet for securing valuables and important paperwork you will want to lock it in with those items.
Here is an article on Windows Client Security and Control you may find of interest. Though it pertains to Windows, MAC users may find some useful tips here, too.