Nothing. You can use a program called Explore2fs (http://uranus.it.swin.edu.au/~jn/linux/explore2fs.htm
) to read from the drive, but there is no write support for windows. (I assume the partition is ext2/3)
You have to backup the data on your drive, and repartition it for a full linux install, or use a live cd anytime you want to access the data, or reformat the drive to ntfs/fat32, or use it read-only, as mentioned above.