On the “work” side of things:
What type of internet connection feeds your work place? Setting up the PPTP Server is the first step. Making sure clients can get to it from the outside is the second. If you are using a NAT router at work, make sure that TCP Port 1723 is forwarded to the internal private address of your PPTP Server.
On the home side of things:
A port forwarding rules does not have to be setup on your home router. All you have to do is make sure that PPTP Passthrough is enabled on the router. On my old 802.11b Linksys router it is under the Security->VPN Passthrough tab. Enable PPTP Passthrough.
Other than those basic settings, make sure that there are no firewalls on either side of the connection that would be blocking TCP 1723.
I should have asked this in the beginning, but can you be more specific as to what VPN error 800 is? What does it say?