There are not many viruses written for Linux, and even if a user downloaded one by email, because the user is by default logged in to a limited account, the virus cannot modify system settings. Only a “super user” can make system changes or add more software by logging in as “root”. Even a computer administrator would normally log himself in as a limited user in Linux and only log in as root to make changes. Though Microsoft encourages people to do the same in Windows XP or 2000, very few people actually do that. And because some Windows software will run properly only if run in an account with Administrator privileges, most users would prefer to be always logged in as the Administrator. They can therefore with administrator privileges damage the system by their own hand! Not so with Linux. All common user applications run perfectly fine in a normal limited user account. System command line tools such as mounting or formatting disks or partitions will only run when logged in as root with the Super User password. This means you can set up a user account for your kids or your mother, and even your enemies with the assurance that they will not be able to hurt your Linux system accidentally, let alone maliciously!
Macintosh computers also have the same security concept of always working as a limited user with the ability to log in as an Administrator from within the user account to make system changes. That’s because Macs are based on Unix just like Linux is!
Windows Vista and now Windows 7 has finally implemented the Unix / Linux / Mac concept of always working in a limited user account with the ability to log in as an Administrator from within the limited account with a password. However I hear that the user cannot delete a shortcut on your desktop without going through a bunch of dialog boxes.