You can create junction points in your FTP home directory that point to the other drives, one per drive. For XP you'll need to download a utility called
linkd.exe to create them.
A note of caution...Don't create a symbolic link on a volume that references that same volume. Bad things happen in XP when you do that. But in your scenario, that shouldn't be a concern as you're creating them on the C volume and pointing them at other volumes.
More info here: http://en.wikipedia.org/wiki/NTFS_junction_point/
If you were using Vista or Seven, then you could use symbolic links, which are a lot easier to work with.