this will let you HFS+ on xp/vista:
http://www.mediafour.com/products/macdrive/ or maybe something from
http://www.macwindows.com/disks2.html
I have the opposite problem - 4 or 5 external drives formatted as NTFS. 10.5 reads them just fine but can't write to them. Was thinking about purchasing
http://www.paragon-software.com/home/ntfs-mac/ Has anyone used it? The MacFUSE developer has abandoned ship so I'd rather not go down that path but I could be convinced...