It's pointless to discuss which OS is better in terms of safety against viruses/malware/trojans because it depends on various factors:

- robustness of OS itself (OS with fewer 'holes' in the system will be safer)

- patching period (if security patches are issued regularly, and installed promptly, the OS will be safer)

- number of users (attacker prefers wider spread OS platform to affect more computers)

- user attitude (careless users will likely get their computers infected even with anti-virus installed)

Statistically, Mac users are safer then Windows users, but that doesn't mean every Mac user will be 'virus-free', and certainly it doesn't mean there are no 'virus-free' users in Windows community.

P.S. I might not be a good example, but couple of years ago, I used Windows XP with no anti-virus (or similar) for about a year (testing it every 3-4 months) and through all that time haven't got a single virus.