From what I've read, I don't honestly think it can be done (at least on a Mac). Or at least not without going through a lot of work, and even then. Your best bet is to just back up and important files on the Windows partition and then delete it to start from scratch.
EDIT: Okay, looks like there may be a way after all. Check the guide here. You might have to buy some software to do it though. And there is the chance the data on the partition will become corrupt from doing it, so you might end up having to reformat the partition anyway.