Well I had some trouble connecting to a Windows Server VPN with Leopard, and found a very simple solution. I don't know if this will work in all cases, but here goes anyway..
Goto System Preferences -> Network. Then click on the little "+" sign at the bottom left saying Create a new service when you hover the mouse over it. Choose VPN for Interface, and PPTP for VPN Type. Add a New Configuration, type in your Server Address, Account Name (This is your VPN username), and under Authentication Settings your password.
When I did all of this I could connect, but couldn't access anything on the network. The fix was to go to "Advanced" and check the Send all traffic over VPN connection option.
After doing all this you should be able to connect and access the network.