Time Machine sharing external hard drive with windows
Time Machine uses the Mac OS Extended file system whereas Windows is using NTFS or FAT.

If you want to use the same hard disc for backuping both operating systems, I would advise you to partition your hard disc, so that you are able to format both partitions with a different filesystem (one for OS X, one for Windows).
You can do this from OS X using the disk utility tool (system prefs).


Format one hard disk with MAC OS Extended (Journaled) and one with FAT (or NTFS directly within Windows).

Now you are able to use the hard disk for backing up both operating systems.