You'll have to configure Windows to act as a NAT router from its VPN interface towards the Internet interface. I see no reason why configuring this would be any different from configuring it for a local (non-VPN) connection. Basically you need to tell it to share your Internet connection with computers on the VPN interface.
If for some reason you are unable to use the GUI, google for "windows xp nat routing" to see how it can be done from command line. Once you have NAT/ICS set up, it's just the matter of adding appropriate routes to the VPN clients:
- A default route via the Windows XP box's VPN IP address. This route needs a metric lower than the regular default route, so it will override it.
- A route to the Internet IP address of the VPN server via whatever was the original default route. This route needs a metric lower than the above added default route. This is needed because the above override for the default route has overridden it entirely - but we still need the connection to the VPN server itself (Windows box) to go directly through the Internet.
You haven't mentioned exactly which VPN protocol you use. I've read that Windows XP supports PPTP and L2TP. I suggest that you use L2TP and not PPTP, because L2TP is Layer 2, and I'm not sure how DHCP would work on a non-Layer-2 network.