I want to have a partition where XP and Mint share files, like Music/Documents/Pictures/Video/ETC
Can't do that.
This is incorrect.
Linux's uses the ext2/3/4 filesystem; Windows cannot see those partitions. Linux, however, CAN see Windows partitions. What you would need to do is mount the Windows partition on startup (autostart). To see Linux partitions on Windows, you need a program called ext2explore which you can find on Sourceforge.
It's true that Windows can't natively read Linux filesystems; however, a cross-OS file-sharing partition is easily created -- just create a separate partition and use a filesystem that both OSes can understand, such as FAT or NTFS. (I favor FAT, since Linux's support for it is faster and more reliable; however, FAT's got a 2 GiB file-size limit that can be a serious problem, so you may need to use NTFS. Other filesystems are options, too, if you provide a suitable Windows driver.)
To use the data-sharing filesystem, you'd access it directly wherever you mount it (you can decide on where that is). If you want to have conventional subdirectories of your home directory point there, you can use symbolic links.
I advise against providing Linux direct access to the Windows boot partition. Linux's NTFS-3g driver is unlikely to be as reliable as Microsoft's own NTFS driver, so giving Linux read/write access to the Windows C: partition can do nothing but increase the risk of serious disk problems. Furthermore, you might accidentally damage files from Linux that Windows would prevent you from touching. IMO, using a separate data-sharing partition is the way to go for best safety.