Symantec puts these out a couple times each year. They are generally informative and this one is no exception. Plus, they have instituted a synopsis for those of us too lazy to read through the entire thing.
There is something surprising in this report. Symantec, though not friendly to Microsoft any more, rates MS operating systems as having greatly improved their security. This applies to server and desktop OSs, and reflects changes to XP and the coming of Vista.
It's worth a read.