Apple makes it tough to add songs from an iPod to iTunes, but I don't think from iTunes to iTunes is hard at all. You should be able to use Apple's method of backing up your iTunes library, which can be used to move your library to an external or secondary HD (I'm doing that as I don't need my 16GB of music on my SSD). Don't know if this will work across OS's on the same HD though. You might have to move to an external with OSX then from the external to Windows. Then tell iTunes the final location to use in Windows.
Here's how I moved mine to my secondary HDD: How to Use iTunes On An External Hard Drive