<rant>I support Sophos. Please, please, please, everyone go out and buy a Mac for you, your friends and all your family members. Let 75% of businesses switch to Mac...please. Then, the virus code writers will target the Mac OS and leave me alone.</rant>

Basically, whichever OS serves the masses of lusers is going to be the one that is targetted. You can point out flaws in MS/Windows all you want, but the fact of the matter is that it boils down to the users themselves. If people knew five simple basics (download the patches, install a firewall, install an anti-virus software, create a Non-priveledged account for daily use, and don't click on anything stupid) they would thwart 90% of malicious code. Additionally, Unix and Linux are just as vulnerable as Windows if they aren't locked down properly. The best thing you can do is lock-down your box and teach all your friends how to lock-down theirs.

BTW, you'll notice that those four steps are NOT OS specific. Those should be done on every box, every patform, and every user.

[Edit] In haste I forgot to add patching...sry. [/Edit]