VPN Setup for a small office.

I'm fishing here in the deeps of the internet because if I catch nothing, at least I can throw myself into the icy waters and drown and end my misery.

My application: Connect remote users (ourselves and our accountant) to my office network to access files, most notably to execute real time accounting software transactions to avoid having to transfer by backups and removable media.

My network/equipment: 1 Linksys BEFVP41 router, with 3 computers currently connected to the router, runningXP Pro, XP Home, and Me respectively. The remote computer could/will be running any combination of the above as well as I don't know the remote users operating systems. For arguments sake let's say they all have XP Pro, and I'll upgrade to it on all systems to implement the solution. My connection is provided via a Cable Modem with a static assigned IP address for the company network, and the home users will be using either the same Cable internet service or a phone company DSL connection.
I'm at my wits end trying to make a connection, and nothing I've tried works. I get error messages(usually error 800) on every attempt. Nothing has worked, and I've followed all instructions in the router's manual as well as copious amounts of Microsoft tech support pages.

Anyone with complete solutions, partial solutions, helpful pointers, or phone numbers of good mental hospitals is encouraged to reply.

Thanks in advance.

Chris