Multi-user and FTP

I don't think FTP is going to work. The database server in QB needs FILE access to the qbw file. If your FTP server can also be setup as, or already is, a file server, then just map a drive letter to the folder it's stored in and use that to open the file from whichever machine will be the host.

FTP is FILE Transfer Protocol. It's for transferring entire files and won't work for remote access.

Remote access is built in to WinXP (see the WinXP help file). That transfers the screen, keyboard, and mouse to a remote computer. For something more exotic, you'll probably want a VPN (Virtual Private Network).

Is there a free file server hosting that allows drive mapping, so I can test the effectiveness of this? My real problem is that I have users in two different cities, using two different ISP's.

Drive mapping is only really feasible on a LAN. Since you have remote users, the only good solution is remote access. If you search on "Terminal Services" on this group you will find a wealth of posts regarding how to set it up.

I have a VPN from home to the office machine. It works to a degree, but even on a database that's only been running from the first of April, things are a bit too slow (on QB 2004)

I find it much more useful to use TightVNC (server) on a PC on the office LAN and log into it over the VPN. This allows full speed database access via local machine and 100% secure logging into the TightVNC machine through the VPN.

As VNC is only updating a compressed image of the desktop (and mouse/key commands) to my home (remote)P.C. the usability of Quickbooks is 90% of using it locally.

If that makes sense.