Need WAN0distributed filesystem... Groove Server?

I have a client with a workforce that are geographically distributed, and as
such, doesn't have a single office where all users are located. They have a
few servers, including an AD domain, and we have been using the Frontpage
Server Extensions of IIS5 (Server 2k3) on their data server to share files
over the WAN, but Windows 7 and the newest version of Mac OS X don't appear
to be working properly with the (admittedly cheesy) FPSE solution.

The way they were doing it, they just added a "My Network Place" to their XP
installations that pointed to http://server.organization.com/, and the web
server on that machine had the FPSE 2002 installed in the webroot, which was
pointed to the root of their data share on that server. Worked well, until
Win7, which now complains about the URL or something (seems to want
/WebDAVroot strapped onto the end, but that doesn't help any)...

So, besides VPNs (which are somewhat sluggish on their WAN connection) and
RDP (which only appears to go so far with this client-- they don't like it
for some reason), what options do I have for a WAN-distributed filesystem?

Someone mentioned Groove Server to me, and after wading through all of the
marketing-speak on the Office site, I'm still not sure if it will do the job,
and I'm not about to pay Microsoft tech support to tell me.

Can anyone help me out with some info on this? Thanks in advance.

Regards,
Jon Heese