Routing & IP addressing

I have a router which users connect to the corporate WAN for e-mail ( this is on subnet255.255.255.0)I also have an NT server running DHCP for an accounting application.(this is also on subnet 255.255.255.0)and thirdly,I have a DSL modem provided bymy ISP which is on subnet 255.255.255.224
How do I configure the desktops such that they could access all 3 services – corporate e-mail, accounting application & internet with one IP setting?