You'd have to back up the data before proceeding with this, because it will reformat your drive. Keep in mind that you won't be able to access it through Mac OS X, once you change the file system to NTFS. It will allow you to use the HDD only with Windows.
Here is a tutorial you can use to change it: http://products.wdc.com/support/kb.ashx?id=A96SII
You'd have to back up the data before proceeding with this, because it will reformat your drive. Keep in mind that you won't be able to access it through Mac OS X, once you change the file system to NTFS. It will allow you to use the HDD only with Windows.
Here is a tutorial you can use to change it: http://products.wdc.com/support/kb.ashx?id=A96SII