What do you you wish to do?
1- Do you wish to use this drive in both, Windows and Mac?
2- This drive was in a Mac computer and you wish to used in a Windows computer, but you do not wish to format because you will loss the data?
If it's number one you can install the NTFS-3G application, then format your drive in your Windows computer. This software will allow you to use your Windows formatted drive in your Mac, thus allow you to use your drive in both OS.
If it's number two, then you can install this software MacDrive ( this is not a free software) this will allow you to use your Mac formatted drive in a Windows computer without formatting the drive.