Edit: Should also add that in all my machines I have UAC turned off although 7's is less anoying then vista'sMissed this post earlier...
I don't suggest having UAC off.
OSX and Linux use the same type of security.
RayquazzaYou have the historical UNIX security model. This is what Vista's model with UAC tried, and failed, to be. Applications, also, usually do not require administrator (root) privileges, while their Windows counterparts do. This is because of historical reasons - running your daily life as root in any sort of UNIX environment is extremely strongly discouraged, by everyone. The end result is that developers are well aware their users may not have root privileges.
In contrast, almost everyone I've ever seen runs Windows using an Administrator account. Developers know this, and thus choose to, for instance, write their program data to the Program Files directory, which shouldn't be used for such purposes (Chrome, for instance, gets it right, as far as I know). This means that applications usually need administrator privileges to install (and sometimes even run), but for no particular reason - developers are just being lazy and not security conciencious.
Why is this important? If a virus enters your machine, for some reason, on OS X (and, for instances, all Linuxes) in all likelyhood you'll be running as an unprivileged user. This means the most damage a virus can do is the same damage you could do - delete your home directory, infect files you own or can write to, etc. It can never, for instance, add a new account to your machine, infect binaries that you can't write to (includes most privileged applications in the system), etc.
On the contrary, if this happens on Windows and you are, like most people, running as an administrator account, the virus can do pretty much anything it wants - it has your implicit permission to roam around free in your computer. This means raising its priority, killing processes (such as antiviruses), infect core system files, etc. This is clearly a much worse scenario than in OS X or Linux.
In escence, it's a problem of inertia - lazy Windows application developers (I'm speaking about third-party applications here, like Adobe's Flash) require administrator privileges needlessly, and this in turn makes your machine much more vulnerable to attacks. In a *nix's case, since it's much less common you'll even have these privileges in the first place, very rarely will applications need root for no good reason, and thus you'll have less places where the virus can do a lot of damage.
Mind you, in both cases, getting a virus and running it implies it has the same privileges as you, so you may have to kiss your data goodbye (the one you can modify). However, unlike in the case of an administrator-level infection, you don't need to reinstall to be secure (if the virus has gotten administrator level privileges, from a security standpoint, you can never trust this installation again - every binary you run may be part of a rootkit. There's nothing you can do that the virus can'tve thought of and planned for - it's a cat and mouse game and you are playing against, very likely, a team of well-trained russian programmers
). You simply need to run an antivirus (and hope it catches the infection) from an administrator account.
If you're curious, the differences in practical usage stem from UNIXes being mostly used, historically, for multiple users at a time, as a time sharing service. In Windows' case, it's been mostly used as a single-user machine (and, in fact, early versions of Windows had no actual concept of user accounts, at best they had different desktops, but it was single-user).
