Click here for ways to use Windows formatted external drives with Mac Systems. It is worth mentioning that Mac OS X (Panther 10.3.x) can now read NTFS Volumes; this means that you can copy files from an external drive (formatted with NTFS) to a Mac running Panther. However, you cannot write files from the Mac to the NTFS Volume.
The following 3rd party software products available that allow external disk drives formatted in the Macintosh file system (Mac OS Extended, HFS+) to be used on Windows systems.
MacDrive from MediaFour
MacOpener from DataViz
NOTE about 3rd Party Software:
Fitness for use in your specific application and support of these products are the SOLE responsibility of their respective publishers.