Depends on the type of virus and if Windows has access to the Mac partition. If it's a cross-platform virus, then you would have the possibility of contracting a Mac virus via Windows. This would only occur if you had a utility such as MacDrive installed in Windows so Windows would be able to access the Mac partition. If you don't have MacDrive or a similar utility installed, then there is no chance of contracting a Mac virus via windows.
In all reality, though, there aren't any OS X viruses in existence. There have been some proof of concepts out there, but nothing that's actually done any damage. Based off of this, there's really not much to be worried about.