Windows cannot (without third-party help) even see that a Mac partition exists, let alone "see into" one. OS X can see and read, but not write to an NTFS partition (and there doesn't seem to be any third party help here, since Microsoft owns NTFS and doesn't license it where it won't make them money). So the solution by default is to use FAT32 for a partition that both XP and OS X need to be able to both read and write.
Now for the extra cool stuff: any external drive can hold anything you want either and/or both operating systems to see (as long as it's in the FAT32 format). Windows itself must be on the internal Mac drive, but its data and programs don't need to be, so you can put all that stuff on an external drive and not clog the internal hard drive with them-which helps the responsiveness of both OSs.