Damned if you do ....
To the guy who is saying mainframes, corporates have the most market share so why don't they get hacked (win vs linux etc) .. These are corporate environments that are secured with multiple firewalls, intrusion detection systems, security at all levels and IT professionals who design and operate it all. Hardly a comparison to your average home user. Sure they're not 100% secure but a damn site more than my granny's router.
Take it back to the desktop and the majority of market share is windows. If the majority was OSX or Linux there would be an equal number of viruses, of that I have no doubt. If you believe otherwise you are deluded. In fact, I would say it would easier with Linux as the source code is staring you in the face.
I've used all the main OS's and my experience leads me to believe a few rules:
1. People who want to get things done quickly - go with windows.
2. Gamers can only realisticly go with windows.
3. People who want to invest the least time in learning computers go with Macs.
4. People who want to invest the most time learning computers go with Linux,
They all have their ups and downs, some are good for certain situations, some or not.
What I would say is that the global recession has certainly boosted the case of open source. The new Ubuntu release has damaged their reputation. Jackalope was spot on :(