This is just a user to user forum so no definative answers here.
Your best bet is to call Adobe and ask. I know you can do it, just don't know the price.
Also, CS6 is set to start shipping on May 6. If you buy CS5 now you will get a free upgrade to CS6.
This is called a cross-grade. Search this forum for this term for further discussion on the topic. You will need to call Customer Service and explain that you need to "crossgrade" from Mac to Windows.