Formatting your hard drive with exFAT is a great way to make it usable between both Windows and OS X. If you want to back up with Time Machine but access your backup from Windows, it's possible to use an exFAT volume with a few tweaks.
Photo by ryanmotoNSB.
Mac OS X Hints has the details at the link below. You'll need to be comfortable with a little terminal work, and we're not sure we'd trust our only backup to something that's unsupported, but it could be useful in a few situations. Hit the link below for the details.
Using Time Machine on unsupported volumes [Mac OS X Hints]