If you mean it's a BootCamp installation and formatted as FAT32, you can just mount the BC partition and drag the files over.
Otherwise (if NTFS) you'll need a 3rd party tool (the name escapes me now), or use MacDrive while booted in Windows to copy from the Mac partition.
edit: Paragon NTFS is one of the 3rd party tools which allows writing from Mac OS-X to a NTFS partition