how to set VPN connection at windows XP

how to set VPN connection with windows XP?
i used windows xp..i want to connect to my office
so i will become vpn client and my office become vpn server.

i was set at my computer :

1. fire wall is off
2. make a new vpn connection

but when i try to connect to my office always said error 800

what else i must set at my computer?

note : my office used router and my client can VPN to my office already.

thank you