The Mydoom virus underscores one key difference between the two systems. OS X comes with a default setting that requires a login name and password before it allows any new software to be installed on a given computer. With this default, no Mac user needs to worry about inadvertently clicking on e-mail attachments carrying virus software. – BusinessWeek
You only need to enter a password to install certain programs, not all. And you don’t have to enter any password to run a program.
Not everything in OS X is secure. The relatively short length of the passwords for accessing individual accounts isn’t a good thing
That was fixed a while ago. I think.
Don’t believe everything you see in print. ;)