A lot changes over time. Especially in a very dynamic computer era.
there was a time when people thought a Rolls Royce car and a Harley motorcycle was the best to have!
And indeed perhaps 20/30 years ago this was the case.
These days the standard for cars, motorcycles and software has increased a lot.
What's good yesterday, doesn't necessarily mean it's still good today.
I think, if Windows 7 is an improvement over Vista, and even XP are very stable Operating Systems that easily can be placed next to MacOs.
Since it's easier to program a Windows program, then a MacOs program, more programs (as well as worms and virusses) had been created for Windows.
Of course, Windows was more affected by lousy programming, since many programs to date on the internet are created by hobby programmers (not so much by large companies).
While the majority of the Apple Software is created by apple programmers.
Windows has more programs than MacOs, which also means it has to have more 'bad' or 'lousy' encoded programs; and has to have more 'errors'and 'bluescreens' than MacOs.
But I think XP is a very stable system!
So it does not impress me if MacOS can be affected by trojan and virusses. It can!
It's just a good way of telling the world that the myth of MacOs doesn't need a virus scanner or firewall is but that.. A Myth..