You have a point here, but that's not really the problem. You see, Linux uses a much different file structure than Windows, such that every file on your PC uses permissions. For example, if a user runs an application, he can only modify a text file if the user owns the the text file as well. Even if a virus landed on your Linux machine, it wouldn't be able to do anything since it doesn't "own" any of your files. The whole kernel would just reject it. Windows doesn't have this; if a virus lands on your machine, it can do anything, anywhere. You're totally defenseless.
While yes, linux does use a very different structure, that isn't what makes it more secure... you can actually do this with windows (If you want to) too. Permissions are pervasive in any os nowadays.
As to viruses on linux, they do exist, but not in the sense that they do on windows. The kernel of linux is more secure yes, but every layer on top of the kernel that is added is inherently less secure than the last. So, the trick to writing a linux virus is to not attack the kernel, instead you attack the GUI (Gnome, KDE, xFCE). It can be done, and proofs do exist for this, however, since linux is such a small portion of the market, it is not exploited the way that would be and is with windows.
As to being totally defenseless in windows, if you take the time, you can lock files down to a comparable level as linux, yes there are still going to be more holes, but thats simply because more people exert their efforts in attacking windows and not linux (Although linux servers are popular choices for C&C [Command and control] Servers for botnets).
Regards,
Ayrus
