Answer:
This question cannot be answered with a simple yes or no. A windows system is more likely to be attacked simply because there are more of them. In addition, windows users may not patch their systems as often as they should. As a result, they "seem" to be more prone to viruses.
If a virus writer knows that the chances of successfully attacking a windows systems are better, then they will try to target the virus to that system. A windows virus generally will not affect a different operating system.