Windows XP is the worlds most widely deployed Desktop operating system. Because of its design, most users run as "Administrator" because that is the default, and it is a pain in the ass to do otherwise. UNIX was developed from the ground up as a multiuser operating system with a clearly defined separation of powers where running as a normal user is fine until you need root, at which point you can easily and temporarily gain privileges when you need them. (Vista has made inroads on this problem, but still has issues.) This means that when you download a file off the net, to install it you have to present a password, which forces the (intelligent) user to think about the repercussions of running code that you just downloaded from *somewhere*. Also, most UNIX-like operating systems (Linux, *BSD, and there is an implementation for OSX [but it isn't as widely used]) use some form of software repository system (apt, yum, ports, etc...) where there is a reasonable expectation that you are getting the 'legitimate' version of the software sans trojans. Honestly, I am having a hard time remembering the last time I needed to 'just download' a binary package since moving to Linux; sure there is the occasional ./configure, make, make install that I have to go through, and sure there could be a trojan in the source, but again it is usually downloaded from mostly trusted repositories that have restrictions on who has commit access. In the Windows world you are forced to download binary packages and hope for the best (Is every rapidshare uploader of that application really out to help you?)
